Vulgar colloquial expression meaning to wipe one's buttocks (or to wipe a diaper/arse). Considered crude in everyday speech.
赤ちゃんのオムツ替えで、ケツを拭くのは大事な作業だ。
When changing a baby's diaper, wiping the baby's bottom is an important task.
高齢者介護では、利用者のケツを拭くことも仕事に含まれる。
In elderly care, wiping a client's bottom is sometimes part of the job.
トイレから出たら、きちんとケツを拭いてください（くだけた言い方）。
After using the toilet, be sure to properly wipe your bottom (colloquial).
Figuratively, to clean up someone else's mess or to take responsibility for the aftermath of a failure (to 'carry the can'). This is a colloquial and coarse expression.
上司のミスのケツを拭くために、深夜まで残業した。
I worked late into the night to clean up after my boss's mistake.
彼はいつもトラブルのケツを拭かされる立場にいる。
He's always in the position of having to clean up after troubles.
取引先との問題で部が責任を取らされ、幹部がケツを拭くことになった。
Due to a problem with a client, the department was held responsible and executives had to clean up the mess.