Rashi: It is adultery with a married woman.

R. Yonah #1: The verse comes to scorn desire. One who is drawn after desire, and at the time of desire does not look at the end. Bread of theft and extortion is pleasant to him, even though after eating it his mouth will be full of sand and gravel, and break his teeth; this is a Mashal for the punishment. This tells man's great folly. They sin to seek pleasure, and they know that in the future, the pain will be due to themselves. One should make a calculation - the loss [in order to do] a Mitzvah opposite its reward, and the gain of a sin opposite its loss (Avos 2:1). Desire overpowers and closes one's eyes from seeing the future punishment. Shlomo gave this Mashal, so man will put to his heart - if ¹ his mouth would become full of sand and gravel, and he will not depart from there after eating bread of Sheker, surely he would refrain. All the more so he should refrain from sin due to the bitter punishment that will come for it!

R. Yonah #2: Perhaps verse 16 explains our verse. It comes to discuss the folly of most people, who sin for the sake of pleasure, even though they know their future punishment. "Lekach Bigdo Ki Arev Zar" - one should not pardon one who does such a great negligence, to trust in one whom he does not know. Stam people steal and extort for pleasure, even though they know their awesome punishment. If they do not heed this due to their Yetzer ha'Ra, why should he trust in this person that he will be concerned [not to do] and guard his Bris?! Just like this verse says about desire, so it says above about laziness - "me'Choref Atzel Lo Yacharish v'Sha'al ba'Katzir va'Ayin" (4). An Atzel chooses rest, and it causes this great pain.

Malbim: It comes via theft, deception, overcharging

Rashi: It is tiny stones, like "va'Yagres b'Chatzatz" (Eichah 3:16).