A 5-card variant of poker. Each player dealt two cards face down. 1 round of betting. 3 cards dealt face up in the 'community' which any player can use. These cards are called the flop. More betting. 1 more card to community, known as the turn or fourth street. Bet again. 5th community card, known as the river. Final round of betting. Best 5 card hand wins. Any combo of the two cards in your hand, and the 5 community cards.
Game has become very popular recently due to ESPN's televised World Series of Poker from Las Vegas. Also, Bravo's Celbrity Poker.
Based on lyrical evidence, this term refers to those that visit a club and, instead of dancing and being sociable, remain out the outskirts of the dance floor, away from the 'action.'
A more traditional term for this behavior is 'wallflower.'
However, as evidenced in 'Everytime Tha Beat Drop' example below, 'holding up the wall' may also refer to a case where an individual requests that another engage in an activity only to be turned down due to (occasionally perceived) social or financial status (i.e. the archetypal 'girl that's too good for you').
"I'm soon to motivate the room ... If you holdin' up the wall, then you missin' the point." -Pharoahe Monch - Simon Says
"Cause I ain't one of these too bourgeois broads /
Always in the club holding up the wall" -Monica featuring Dem Franchize Boyz, Everytime Tha Beat Drop