“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.

I don't get it.

This 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 also more casual than "I don't understand."