A player is someone who plays a role-playing game. For instance, if you and your cousin Edith decide to play Dungeons & Dragons, you and Edith are the players. Players who play a game together at a table are a group. Usually, a game also has a GM, who is a player with slightly different responsibilities (but is also sometimes not called a player, depending who you ask).
In most games, a player controls (makes decisions for and manages their traits) a character called a player character (PC).