During any given tennis match there will be an over/under number set on the number of games that will be played during a tennis match. Typically, if the feeling is that the match will remain close throughout, then the over/under number will be larger as the number of games will be more in a close match. However, if the match is projected to be a blowout, then the over/under amount will be lower as the number of games in the match would be lower.
The given 21.5 amount is the over/under for the specific match that this is talking about. If you were to bet on the over 21.5 for this match, then you would be saying that you think the amount of games in this match will be 22 or more. However, if you were to place a bet on the under 21.5 you would be saying that you think that the number of games in this match would be 21 or less.
The over/under for every match will be different and will depend on who is playing and the projection of the match itself. Betting lines are created in order to get half the bets on one side and half on the other so the lines will shift depending on the amount of bets being placed on each side.