What Does HMM Means? 

Most people used HMM when they completely understand what someone trying to make them understand. If they Completely understand they use HMM it is a Reflection & Confirmation to inform the other part that all the confusions are clear and totally understood.

HMM stands for? 

When to use HMM?

HMM is used when you want to tell somebody that all confusions are clear on a particular matter and there is nothing more left to be understood.

Is using HMM Formal or informal?

Yes, It is totally Informal some peoples mostly use this acronym and thinks that it is formal but unfortunately using HMM in our daily life is informal.