CG stands for challenge games. This means the number of games that they have challenged other players in this ladder.
Remember: you can challenge a maximum of two players in a ladder at the same time. If you've already challenged two players, you have to wait until one of your games finishes before you can challenge another player.
AG stands for accepted games. This means the number of challenge that they have accepted from OTHER players.
Remember: a player is only obligated to accept 2 games from other players. After that, they cannot accept any more challenges in that ladder. So if a player has already accepted 2 games, then you won't be able to challenge that player until one of their game finishes.
This column shows whether or not you are allowed to challenge that player. If you are allowed to challenge that player, then there will be a link labeled 'yes'. Click that link to challenge that player. If not, the column will say 'no', and you will not be able to challenge that player.
Vac stands for vacation status, and it shows whether or not that person is on vacation. A 1 means 'yes' the player is on vacation, and a 0 means 'no' the player is not on vacation. If a player is on vacation, then they cannot accept any new challenges.