In stitches means when somebody is laughing so much that it is difficult to control themselves. Examples: The movie will keep you in stitches from beginning to end.

Charlie had us in stitches with all his jokes. (to have somebody in stitches - to make somebody laugh uncontrollably)

Note

To make stitches - sew, knit

a stitch - A single complete movement of a threaded needle in sewing

A proverb

"A stitch in time saves nine" - A timely effort will prevent more work later.