Just like in real-life football, Hattrick players can receive cards when they make faults. When a player receives a red card in a competitive match, he can't play in the following official match (cup or competition). This is also the case when a player collects 3 yellow cards in several competition or cup matches. When a players receives a card in a friendly game, then this card isn't important for the following matches. It just has his consequences for that match itself.
At the end of the season, the cards are reset.