This word is usally used to say 'sure', 'yes', or 'okay'. sometimes you use it..in a sarcastic was being like 'sureeee i believe you'. I sometimes use it when there is just nothing to say.
Used as a casul response when you are fed up with someone speaking, to let the personassume you are following up with them but honestly...you're really not, and you really don't care.
person 1: guess what, I passed my exams and got a brand new car!
person 2: Mhmm. that's nice I guess.
or
person 1: I come with some terrible news....my dog just died
person 2: Mhmm....that's nice. glad to hear that.
person 1: wait what!? so you really don't care?
person 2: mhmm. oh-wait--what?-wait-shit-damn-well-hold up real quick....what were you talking about?
person 1: you know what, nevermind. don't know why I even told you when you won't even listen to me.