Eminence is an ability found on certain Commander-focused legendary creatures that functions while the card is either in the command zone or on the battlefield. This makes it unusually powerful, because you get the benefit before you ever cast the commander—just having it sit in the command zone grants the effect.
The most common mistake is forgetting that eminence works from the command zone. For example, Edgar Markov's eminence creates a 1/1 Vampire token whenever you cast another Vampire spell, even if Edgar himself is uncast. Players also wrongly assume the ability stops if the commander dies—but it returns to the command zone, so eminence keeps working.
In Commander, eminence shapes deckbuilding around the commander's tribe or theme. It's a reliable, removal-resistant engine since opponents can't easily shut it off without exiling the commander.