Shortly after an adult John Connor emerged from his bunker in 2009 after a nuclear winter, he came under the attack of Skynet and the Terminators. John found himself under the gun of a Terminator when he was saved at the last moment. The Terminator was shot in the head and destroyed. Looking up to see who his savior was, a leather-gloved hand reached out and said, "Come with me if you want to live." Seeing a huge bald "man" in a leather coat and dark sunglasses, John asked who he was. The "man" told John he could call him, "Uncle Bob." John's savior "Uncle Bob" is actually a terminator sent by John Connor's future self. Uncle Bob's role is to help John get over the loss of Katherine Brewster and become the leader he was born to be. Skynet sent the T-Infinity prototype to eliminate Uncle bob and correct anomalies in the timeline that will lead towards Skynet's destruction.
Uncle Bob is the name the young John Connor gave his T-800 protector when introducing him to Enrique in Terminator 2: Judgement Day.