a) One's plans or intentions, as made popular by a recent Black Eyed Peas song by the same name.
b) one's inclusion is the Superfamily Apoidea, a major group within the Hymenoptera Order.
Stacey: Imma be up in them A-list flicks
Doin' one-handed flips, and Imma be sippin' on drinks.
Bee: Imma be! (Buzzz, buzzz, buzzz.)
"I'm going to be"...OR "I may be"...
Hence, it's a shortcut, specially used in Instant Messaging, texting and/or eMailing in order to conserve a long sentence thus saving time specially for those who are non-typists.
Immabe busy tomorrow.
Immabe looking for a job.