1. Used as a declaration of your extreme indifference.
2. When you can't decide whether something is good or bad, you meh.
OMG, They put the cheese INSIDE the watermelon!11!1one!
ON A SCALE OF 1 - AWESOME, HOW AWESOME IS IT???/
When you just don't care about the subject.
John: Do you want a blue lolly, or a red one?
Lilly: Meh. I really don't care!