Relegation is when a team moves from their league into a lower league. It is the opposite to promotion. For example, a team in the English Premier League that is relegated will be sent to the Coca-Cola Championship. If they are relegated again, they move to Coca-Cola League One.
The positive thing about relegation is that the teams in the lower league will easier to play and there is a good chance that the team can win or do well in the league. However, this is rarely a consolation for the club and its fans, as reaching higher leagues is one of the main goals of a football team.