A two-minute Google returns the following:

From Taylor Guitars:

"A truss rod is an interior metal bar running the entire length of a guitar's neck. When tightened, it counteracts the tension created by the strings (the strings pull the neck forward; the truss rod pulls the neck backward). Proper use of the truss rod enables you to "balance" the tension on the neck and adjust the amount of bow to fit your playing style."

See also Frank Ford's Frets.com article on truss rod adjustment.