Kata is a preset series of movements. It has some similarities with a dance. But in kata you act as if you fight two or more attackers. The attackers aren't there, but you do react to the attacks, even if they only exist in the mind. Kata is imagination. But kata is also a great tool to learn karate techniques. Any kata contains a great variety of techniques. All the katas combined form a vast encyclopedia of karate techniques.
One of the difficulties of performing a kata is to keep acting as if you really get attacked. If you don't your kata will turn into a pointless series of movements. Some say: a dead kata. Always try to keep your kata alive. Don't keep repeating dead kata, but try to do a live kata better each time.
No one in the world can perform a perfect kata. Some people can do better than others, but no one is perfect. That's a simply a fact of life. The good part is that you can always keep improving, even after ten thousand times. It's fun to keep getting better. And the fun never stops.
Here's an example of a kata. This is kata Chinto performed by Hideho Takagi, a senior Japanese sensei.
Click on the picture to start the video (1Mb)