Champion is an ability found mostly on Lorwyn-block creatures (like Champion of the Parish's tribal cousins, e.g., Mistbind Clique). When a creature with "champion a [type]" enters, you must exile another creature you control of that type. When the championing creature leaves the battlefield, the exiled creature returns. They're effectively linked—you "ride" one creature on another for a boosted body.
The big nuance: the championed creature is exiled, not sacrificed, so it dodges board wipes while hidden but won't trigger leaves-the-battlefield or dies effects when championed. If the champion creature dies, the exiled one returns as a new object—summoning sick again, and any counters or auras are gone. Also, if you can't (or choose not to) exile a creature, the champion is sacrificed.
In Commander, watch the timing: flickering or bouncing the champion brings your hidden creature back, which can chain ETB value or protect a key creature from removal temporarily.