1.an expression having a discretionary undertone and is usually used in a state of confusion, or as an aid in connecting lies or other words into informal grammatical conversations; a word that assists in a person’s conversation.
2.usually used by tweakers to finish up their elaborate lies.
Um.. is used most commonly when words are accumulating into your thoughts.
But becareful. In a far off country... out there near Russia "um" means cunt.