eating out is when a man, or in some cases a woman, performs oral sex on a woman, by placing his/her tongue/lips on the genitals (vagina, clit, pussy) and licking or nibbling on either. This is the easiest way for most women to experience an orgasm.
My girlfriend was moaning in pleasure as I ate her out. - Past tense.
I can't talk now, I am eating out, and she loves it. - present
I can't wait to eat out my girlfriend, usually a bj or intercourse follow - Future