Pin the blame on the bad guys and march off to war as patriotically as possible.
This short and sweet resolution by Congress claims that communist North Vietnam attacked U.S. ships, and uses that aggression as a clear reason for retaliation. The President of the United States (that'd be Lyndon B. Johnson) was given the full support of Congress in terms of funding and resources to, uh, deal with the situation and keep Southeast Asia safe.
Bottom line: the Tonkin Gulf Resolution is a declaration of war in the form of a presidential permission slip.
The United States officially entered the Vietnam War because its military ships were attacked, it desired freedom for the people of Vietnam, and it believed it was acting in the interests of world peace.
The United States used a questionable attack against its military ships as an excuse to combat the spread of communism in Southeast Asia.
"The Vietnamese have fired on our ships? That's it, we're going to war."
Not a direct quote, but that was the feeling in Congress after the Gulf of Tonkin Incident where there were reports of Vietnam attacking multiple U.S. ships. Congress whips up the Tonkin Gulf Resolution in a speedy nine hours, and—boom—America is off to war.
Short and sweet, the Tonkin Gulf Resolution is only a single page. (Gotta hand it to Congress, as some of their legislation can have page numbers in the thousands.) But that's kind of the point here—this document is pretty close to a declaration of war, and it was meant to be put into action right away.
The text does three main things:
Patriotic, forceful, and no nonsense. U-S-A! U-S-A!
If you mess with our ships, prepare to face the full force of America's military might.