Basically, it gave President Johnson the power to do anything he saw fit to protect American interests in Vietnam. It meant he could take all the actions of a war, without declaring war, and that's how the Vietnam War started. (Well, one could argue that JFK started it, because he was the first President to send advisors. Or that it started even earlier with our support of France's war in Vietnam (Yeah, we carried them, once again). But, the Gulf of Tonkin Resolution gave Johnson the power to send troops, drop bombs, etc...)