A rounder is synonymous with a grinder. These are players who make their living or a significant amount of their income from playing poker. They tend to play mid or low stakes games and win money bit by bit. Rounding is often considered to be a difficult way to make money because rounders tend to not make much money per hour.
A tough, knowledgeable player who earns his living at the poker table. Often refers to a skilled player who represents himself as being rookie for the purpose of trapping and breaking the other players.