From Wikipedia's article
Roland Kickinger as the prototype T-800: This will be the first Terminator film in which Arnold Schwarzenegger does not play the Model 101 Terminator, because he is serving his second term as Governor of California.[16] Nevertheless, he granted the Halcyon Company permission to use his likeness in the film.[17] The Austrian-born actor previously portrayed Schwarzenegger in a 2005 biographical film on the A&E Network. When asked about his role, Kickinger said it's "Arnold's character in the first Terminator. That's basically my role, but 20 years before, so it establishes how the Terminator [came] about." He also revealed that there is a "very strong scene in the film where John Connor for the first time meets the Terminator, and he doesn't know if he's a good guy or a bad guy."