The meaning of I eat ass is the using of the tongue and lips to stimulate another person's anus.
I eat ass means that the person who eats ass performs an anal and oral sexual act called Anilingus.
Anilingus is an oral and anal sex act where one person stimulates the anus of another person by using their lips or tongue.
Simply put, rimming, or anilingus, is using your tongue and lips to stimulate another person's anus.
So if you like to use your tongue and lips to lick and stimulate someone else's anus or ass then you like to eat ass.
A woman may eat a man's ass or a man may eat a woman's ass or a man may eat a man's ass or a woman may eat a woman's ass.