But what does it mean?

Like any system, CATCH can't be the perfect solution, nor does it cover everything an umpire needs to know. But that it's easily remembered is a good start. If you're mentoring an umpire on the field and you try to tell him five or six things he needs to improve on, he may remember one or two. It's much the same in the classroom. Teach a group of new umpires a whole slew of procedures, and they will probably retain less than half. But if you teach them what the CATCH acronym stands for and explain it, there's a good chance they'll remember more.