mm hmm

a sound that describes the following:

yes'um, ok, yea

mm hmm

a sound in written form meaning agreement, 'yes', can also be used as a coy/sexy way of agreeing

1) 'you really like those chips dont you?' 'mm hmm.'

2) 'yo baby, you like when i touch you liek that?' 'mm hmm.'

mm-hmm

A sarcastic way of saying sure. Can be used to show doubt to another person.
Mm-hmm can also be followed up with "Sure you do" to show that you really don't believe them.

Guy 1: Hey! I have a girlfriend!
Guy 2: Mm-hmm. Sure you do.
Guy 1: Dude, I'm serious!
Guy 2: Mm-hmm. Sure you are.