We often translate the word "kata" as "form". As is often the case there are many other ways to translate it, depending on context. But for our purposes in the martial arts, "form" seems to be agreed upon.
But what is a form? If I say "form", this being April most American adults are likely to think of a piece of paper -- or a computer screen or web site -- where we put in a bunch of numbers and figure out how much money we owe to the government. Or we might think of form as being the external appearance of things, sort of the polar opposite of substance.
But there's another meaning for "form". When you're pouring concrete, for example, you build a frame out of wood to hold it in place. If you didn't do that, the concrete would just slump over and spread out all over the place. It wouldn't take the shape you want.
The frame, the form, acts like a mold and gives shape to the material. The old kanji for kata -- 型 -- includes the ideogram for earth, because the most primitive way to use this sort of form is in packed earth construction.
In the arts that use kata -- which includes not just Japanese and Okinawan martial arts but also things like kabuki and chado (the tea ceremony) -- *you* are the material.
A kata is something that shapes your practice, shapes you. That's important. It's not a static thing that exists independent of you. When you do a kata, it acts back on you. It literally shapes you. If you keep doing a kata, it will strengthen certain muscles, change your nervous system, train your brain to respond in certain ways.
But the only way a kata is useful is when you do it. Talking about a kata doesn't shape your karate. And it's not enough to have memorized it. That's just knowing the shape of the mold. It's no good to have the form over here, and the substance, the material, over here, outside of it. You have to put the material -- that's you -- into the form -- that's the kata.
With concrete, we do that once, and let it set for a long time until it hardens. But people are a little more complicated! So we have to keep going back into the form. We have to keep doing the kata, so that it can shape us. Over and over, for years.
Without that, our training just slumps over and spreads out all over the place, and doesn't take the shape we want.