What does climbing ash mean?

I don't even know how to climb ashes. Ignorance is terrible. You said such an elegant thing ... climbing the ashes was a matter for Su Dongpo, a great writer at that time. It seems that Su Dongpo saw his daughter-in-law taking a nap in the study. There was a lot of dust on the table. He wrote on the table: "There is a pipa in Furong's account, which is disrespectful to play." Daughter-in-law woke up and answered, "I'd like to borrow a lute, so the rich water won't flow to outsiders." Su Dongpo returned to his study and read two more lines. When the son came back, he saw his father in the study and asked him what he was doing. Su Dongpo quickly erased the words and said, "Climb the ash, climb the ash."