A slogan from a commercial that ran for most of the 1990s, for the Texas Beef Council.
The commercials were voiced by actor Sam Elliott (not tough guy Robert Mitchum, as I've heard some people say) and featured various beef-laden dishes being laid out by stereotypical American families, mostly made up of two parents and two kids, all of whom were practically melting in an orgy of beefy anticipation as their dinner was laid out before them.
While Aaron Copland's "Hoedown" played in the background, Elliott would narrate the dinner scenes unfolding. He would meaningfully intone things like, "Whether you're in Texas having a steak sandwich with Texas toast, or in the San Francisco valley cooking up a barbequed shish-kabob... beef, it's what's for dinner."
The ad campaign cost $25 million in all, and introduced the beef industry's new line of easily-preparable beef-centric products.
The phrase Beef, It's What's For Dinner has turned into a meme of sorts, where "Beef" is interchangable with whatever the task/situation at hand dictates.
Sources:
http://www.txbeef.org/faq.php3
http://www.int-brangus.org/brngsjrn.htm