Hmm is a general ok signal in our general talkings to friends, colleague our relative our close family friend and the people whom we generally talk in common and the whom we love in our daily life it is a general guesture of saying ok procede further while having some general discussion on a general topic when we normally meet our friend and we just generally ask about their life and all is good in their life or either our friend is facing some general problem by talking to our friend we cand find the solution and while in general talking if he admits there is a certain kind of problem persist continously he then sets signal ok by saying hmm.Also while talking in mobile phones people with long talks and discussion regarding a particular topic they generally say hmm confirming yes to that particular discussion also while chatting to a particular person people generally type hmm when they are busy but have to  reply to continue the chat in progress abd move forward they type "hmm" i.e yes  in reply to the particular asked question.