Hear a lot about importance of metrics; with animation, what does that mean?

If you have a chair and your character has to sit on that chair, and you have one that's 25cm high and one that's 50cm high, you can't have an infinite number of animations to sit on all these chairs. You need just one chair height. Every character in the game will sit on chairs at the same height, and everything in the game needs that. Everything needs a defined measurement.