When playing in a tournament What does it mean by points available? Does it mean points added to your rating or points for how involved you are in Chess.com?

Neither, that is referring to the total number of "tournament points" that are given out in the tournament. These points are split among the players in the tournament, with the lion's share going to the winner. They essentially a measure of how well you have done in Chess.com tournaments in the past. For information on how they are calculated, you can go here: http://www.chess.com/tournaments/help.html?S=1609&A=2&G=6&T=18#points .

So it determines how good you do in tournaments?

It gives an indication of how well you do. But, it can't tell the whole story. If you play in 35 tournaments, but do poorly in all of them, and I only play in one, but win it, you still may accumulate more points than me.

Okay, I understand it. Thanks.