Simply means "Yes", "I agree", or anwsering to a call.
Dude1: HEY!!! DUDE2!!!!!!!!!!
Dude1: Whats up?
Dude2: Nm you?
Dude1: Just waiting for the Playstation3 to come out.
Dude2: Yeah i know...
Dude1: Man, I cant wait till the Playstation3 is FINALLY out...
Dude2: Yada i know, only 3 more days!!!
Dude1: Oh my GOD!!! The expence is way extreme.
Dude2: I know!!!! Ive been waiting for 2 months, thats along time.
Dude1: yeah it is.