This is a euphemism. Alternate translation: "Absalom had sexual relations with his father's slave wives" (See: [[rc://en/ta/man/translate/figs-euphemism]])
This means that people were able to see the tent and Absalom walking in and out of the tent with the women. The phrase "all of Israel" is a generalization, for only people near the palace could see it. Alternate translation: "where the Israelites could see him go into the tent" (See: [[rc://en/ta/man/translate/figs-hyperbole]])