(someone) is in stitches
When someone is "in stitches" it means that they are laughing a lot, and can't stop laughing:
A: How was the comedy show?
B: Oh man, I was in stitches!
If something is just a little funny, you don't use this phrase. You can say something else, like:
It was funny.
I got a good laugh out of it.
The word "stitches" comes from a medical treatment. When someone gets a large cut, a doctor will sew the wound to keep it closed. Sewing together a wound is known as "giving (someone) stitches". It's a little hard to connect "stitches" to laughing, but you can imagine laughing so hard that your stomach starts to hurt as if you had stiches in it.