Hah - The person thinks a comment is mildly funny.
Haha - The person does not think the comment is funny, but acknowledges your attempt humor.
Hahaha - the person thinks the comment is funnier than average, or is just more enthusiastic than those who normally say haha.
Hahahaha - The person is just being excessive. Usually used to
strengthen a friendship because
an inside joke was just mentioned.
Haha is
almost never used to express actual laughter occuring. For those circumstances, rofl, lmao, or a whole sentence about how the comment actually made one laugh is used.