i don't understand why it translates like that... i know "laisser" means to leave, and "tomber" to fall. is it just some sort of expression?
This is a colloquial expression. "Laisser" also means to let something happen, so the end of the sentence means "let it fall". Similar to English when people say "drop it" to end a discussion.
Which is why I think they should have allowed the translation "you have to drop it"
Where is the "it" in the sentence?
laisser has a tacit 'it' too
"il faut ..." is usually an expression that means "it is necessary ... " or "one must ..." so this sentence becomes a combo of expressions for a colloquial one.