A yellow card in soccer indicates a warning for unsporting behavior or a minor foul committed by a player. It serves as a disciplinary measure administered by the referee. If a player receives two yellow cards in a match, it results in a red card, which means ejection from the match. The yellow card system encourages fair play and adherence to the rules while allowing referees to maintain control over the game. Accumulating yellow cards can also lead to suspensions or other penalties, impacting a player's future participation in matches.