It means "Hook 'em Horns", which is another way to say "I love you" in Texan.
It's not a Hook 'em Horns. I know. UT '77. A lot of the teams around here have hand symbols. There's the Hook 'em Horns, when Texas scores a touchdown. The thumb up "Gig em Aggies", for when A&M scores a touchdown, the claw "Sic em Bears" for when Baylor scores a touchdown (usually only seen about twice a year), and several others. This is supposed to be a pirate hat. Hey, there's only so many hand symbols you can come up with without being obscene.