Introduced in the Lorwyn block, it is used to great thematic effect as you give up a creature in exchange for gaining this more powerful creature.

702.69. Champion

• 702.69a. Champion represents two triggered abilities. "Champion an [object]" means "When this permanent comes into play, sacrifice it unless you remove another [object] you control from the game" and "When this permanent leaves play, return the removed card to play under its owner's control."
• 702.69b. The two abilities represented by champion are linked.. See rule 407, "Linked Abilities."
• 702.69c. A permanent is "championed" by another permanent if the latter removes the former from the game as the direct result of a champion ability.

Rules Translation

• 702.69a. - This is a keyword which actually is "overloaded." It represents two triggered abilities.
• 702.69b. - If one of these effects doesn't happen, neither do.
• 702.69c. - This is game rule speak for saying that the 'Championing' is accomplished by removing it from the game.