Um.. is used most commonly when words are accumulating into your thoughts.
But becareful. In a far off country... out there nearRussia "um" means cunt.
You don’t just look up “um” on U.D. So you probably looked it up for one of these reasons
1: you have no social life and was bored. 2. You were looking up something highly inappropriate and someone walked in 3. You are really weird 4. You are a boomer and thought this was a real dictionary which it is not 5. You are a alien trying to discover the human life form