"Mhm" is a casual way of saying "yes" or "I agree" in text messages and social media. It comes from the sound "mm-hmm" and can be spelled in different ways, such as "mhmm" or "mm-hm". It can also be used to express acknowledgment, understanding, satisfaction, or to signal your conversation partner that you are following what they are saying and they should continue speaking - depending on context and tone. In general, "mhm" is a quick way of responding positively online or in texting. It is an interjection and can be used by itself to express an emotion.