When you gab, you chatter on and on. You might like to gab on the phone with your friend after school.
To talk and talk, especially about gossipy or trivial things, is to gab. Your dad might gab over the fence with the neighbor, and you may sit in the front of the bus so you can gab with the friendly driver. You can also describe the chatter itself as gab. The oldest meaning of gab may come from thirteenth century Scottish dialect, "to reproach," although it may in fact be rooted in the Old French gobe, "mouthful or lump."