Um is used in conversational English to hesitate. You use this when you aren’t ready to give an answer.
Often after saying um the speaker is vague and avoids giving a direct answer to the question.
This is called stalling for time.
See STORY example: Ummm… I don’t know.
This is spelled with three m‘s so the reader will make the sound last longer, as though the speaker is thinking.
Uh means the same thing as um.
Because these are both informal, conversational expressions, the spellings may vary.