To have a meal; to eat (colloquial, often rough or masculine tone).
今日は忙しくて昼にちゃんと飯を食う時間がなかった。
I was busy today and didn't have time to properly eat lunch.
うちの父はいつも『めしを食べろ』と言って急かす。
My dad always tells me 'eat your meal' and rushes me.
久しぶりに家族そろって飯を食った。
For the first time in a while, the family ate together.
To make a living; to earn one's bread (used figuratively for earning income).
フリーランスで飯を食っているので、収入が不安定だ。
I make a living as a freelancer, so my income is unstable.
彼は画家として飯を食っていけるようになった。
He has come to be able to earn a living as a painter.