“I don't get it.”
You're talking with a group of friends. One of them said something, and everyone suddenly started laughing. You don't understand why they're laughing, so you think to yourself:
I don't get it.
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I don't get it.
"I don't get it" is a way of saying "I don't understand." You say this when you don't understand something like:
- a joke
 - an explanation
 - why a character in a movie acted in a certain way
 
You probably wouldn't say "I don't get it" if you don't understand things like:
- what someone said (meaning the words, not the content)
 - an entire subject, such as Physics
 - something that's never been explained to you, and that you haven't even tried to understand
 
"I don't get it" is more casual than "I don't understand."
