Junior is the affectionate nickname, first coined by Colonel O'Neill, referring to the Goa'uld symbiote which Teal'c carried. A symbiote acts as the immune system for a Jaffa. Teal'c had replaced his symbiote once since coming to the SGC, after giving his own symbiote to his son. When Teal'c and Bra'tac survived an ambush and were kept alive by sharing the regenerative powers of a single symbiote, all three were found too weak to survive. Teal'c and Bra'tac began using regular does of tretonin to replace their immune systems, and Teal'c no longer carries Junior.
Cross Reference: Goa'uld, Jaffa, Teal'c, Tretonin
Episode Reference: Thor's Hammer, Bloodlines, Message in a Bottle, Holiday, Fair Game, Demons, New Ground, Enemies, Threshold, The Changeling