What does it mean to 'understand' something?

When we know the meaning of things,place at that time we use understand. When you say 'understand' it means you heard everything and your mind process well what you heard or listen.This word 'understand',most of the time it uses in classroom when a teacher teaches to student and when anybody is explaining about a particular thing at that time the receiver uses understand.When you use understand it seems you listend well and it goes good impression to another person.