A "relief pitcher" is a baseball player who comes into the game later to replace the starting pitcher. They help the team by relieving, or taking over, the pitching duties and trying to prevent the opposing team from scoring. Full definition
He is the first relief pitcher in history to hit a triple in a postseason game. (sbnation.com)
In this case he need not be credited with the victory and it is up to the scorer to decide which other relief pitcher should be credited with the win. (si.com)
The negative scenario is an expensive relief pitcher or dead money. (sbnation.com)