The term "hmm" is an informal vocalization often used to express thinking, hesitation, or uncertainty. It can indicate that someone is considering something carefully or is unsure about what to say or do next. This sound is commonly used in conversation to signal that a person is pondering a question or trying to find the right answer.
The exact origin of "hmm" is unclear, but it has been used in the English language since at least the 19th century. It is thought to be imitative, representing a sound made when someone is deep in thought or unsure.