We all know people who train consistently - maybe too consistently - by doing the same thing in the gym every workout, time after time. They might take a break for a holiday or vacation or because of injury, but they generally repeat the same type of workout - the same exercises (generally), similar rep schemes, similar number of sets, and similar tempo, month in and month out. If they're smart they'll constantly strive to add weight, or reps, to their workout, or switch up the exercises once in a while. You can certainly make progress with this style of training, but it's usually slow and boring and often leads to staleness and/or injury.
High level athletes realized a long time ago that if they wanted to peak for an event - say, the Olympics, or even a season of their sport - they couldn't train the same way all the time. They'd do general conditioning for part of the year, then more specific training, and finally sport specific training for the last couple of months leading up to their event. It didn't take long for them to realize that over the long term they'd make better gains by "mixing up" their training this way than if they did similar types of training every week over the year.
Periodization is a term that means changing aspects of your workout in a planned way in order to maximize long term progress. Reading the literature on periodization can be kind of daunting to a beginner - articles about periodization are often filled with technical vocabulary that you really don't have to know in order to use the principles.
So I'll make it simple. I'm not going to define a million different terms for you (partially because I can't remember the difference between conjugate and concurrent periodization), just cover the basic concepts so you can use periodization to improve your training (and you should!).
There are 2 general kinds of periodization: Periodizing physical qualities and periodizing intensity.
You can periodize your training to focus on different attributes or periodize your intensity. Or you can (and should) do both! I'll address the first type here (for no particular reason), and the second type in the next post.
Periodizing Attributes: What this means is that you emphasize different physical attributes in different training sessions. For example, you might do hypertrophy (hypertrophy means muscle growth) workouts (8-12 reps per set, 3-5 sets per exercise, moderate or slow rep pace, moderate rest between sets, extra calories after the workout), speed/ power workouts (plyometrics, very fast movements, ballistic exercises like kettlebell swings or olympic lifts, sprints, 3-5 reps per set, 2-3 sets per movement, lots of rest between sets), strength workouts (4-6 reps per set, lots of weight, moderate rep speed, 3-6 sets per movement, lots of rest between sets), and conditioning workouts (energy system training) (circuit training with moderate weights, little to no rest between sets, lots of sets). None of these workouts are easier or harder, by nature, than the others, they just each target a different type of adaptation.
You could work these into your program in various ways. You could do 3-8 weeks of one type of workout (say, a session of hypertrophy), followed by another "block" of 3-8 weeks focusing on another (say, strength workouts). The potential downside is that you could lose too much in one area while focusing on another - you might lose all the speed you gained during your speed "block" during the other blocks, since you might go 16-24 weeks without training for a particular quality.
You could try to avoid that by mixing and matching in various ways. For example, you could do a little bit of hypertrophy, a little speed work, and a lot of strength work for 8 weeks, then a little hypertrophy, a little strength, and a lot of speed work for the next set of 8 weeks, and so forth. Think of it as having two minors and a major in each block - you'd do enough in each minor area to maintain your ability and you'd make progress in the major area.
Yet another style of periodization (you can see why there's so much terminology around this subject - each method of periodizing has its own name and associated jargon) would be to alternate workouts over the week but focus on each quality equally. For example, suppose you work out three times per week. Do one session of hypertrophy work, one session of speed/power work, and one session of strength work. That way you make consistent progress in all areas.
Which system is best? I think a lot depends on what you're training for. If you're an Olympic athlete who has to "peak" at a certain time of year you need a very different system than regular people - and you probably have a professional coach to help you plan that all out. If you're just like me - someone who wants to keep improving, but has no specific targets - then I'd say try the last system. This is also a situation where we're splitting hairs - a professional athlete, who is in a situation where a 2% improvement could mean the difference between winning and losing, has to be much more meticulous in their choices than a weekend warrior. For us amateurs a simpler system that is easier to comply with is probably more useful than something that could serve as a Ph.D. thesis in exercise physiology.
The "attributes" in question can also vary. Bodybuilders alternate periods of time when they "bulk up" (add bodyweight - usually a combination of fat and muscle), then "lean out" (lose bodyweight, again usually a combination of fat and muscle, but they're hoping to gain more muscle than fat, then lose more fat than muscle, and come out ahead). And you can focus on different attributes for different areas of your body - you could combine strength for the lower body with speed in the upper, and vice versa.
I'll give my recommendations on how to periodize your training - assuming you're a karateka with an actual life - with specific examples and a training plan - after I cover periodizing intensity.