It's used when talking to a person as an answer and sometimes used when you simply don't care for example Girl: Hey I've got new shoes Boy: Mhhh (Secretely doesn't care one bit)
Bob- You like my shirt?
Bobs girlfriend- mhhm...
Bob- Mhhm? Sure. I know you hate it.
Bobs girlfriend- What? Oh, that... Not really. Sorry. I was remembering last night. mmm... :)
Bob- Do you wan't a repeat performance?
Bobs girlfriend- MHHM!