What is Commend in CS:GO?

It is a system that allows you to tell the other player that he is. There are three options: Friendly, Good Teacher & Good Leader.

How can I commend another player?

When you are playing (in the match) then press TAB. Then (press mouse 2) on Player, that you are Commend on.

How can I see that someone has made a Commend to me?

You can see that at main menu.