Bonjour Megan,

"Faire les 400 coups" means "to get up to some shenanigans / to go a bit wild". It is used to describe someone (usually kids / teenagers) who are going to get up to some mischievous silly things (as kids do). ;-)

A couple of good dictionaries for finding French expressions are : Wordreference or Collins.

I hope this is helpful. ;-)

Bonne journée !