This verse describes a deeply shocking and symbolic act of rebellion and usurpation of power by Absalom against his father, King David. Here's a breakdown of its meaning:

"So they spread a tent for Absalom on the top of the house...": This refers to setting up a private space for Absalom on the rooftop. In ancient Israel, rooftops were often used for private activities.
"...and Absalom went in to his father's concubines in the sight of all Israel.": This is the key part of the verse. Absalom publicly had sexual relations with his father's concubines (secondary wives). This act was loaded with meaning:
Usurpation of Kingship: In the ancient Near East, inheriting the harem of a defeated or deceased king was a symbol of claiming his throne. By taking David's concubines, Absalom was symbolically declaring himself the new king.
Humiliation of David: The act was a direct and public humiliation of David. It demonstrated a complete lack of respect and aimed to strip David of his dignity and authority.
Defilement: The act also served to defile David's property, thus further eroding his power.
Publicity for legitimacy: the fact that this was done "in the sight of all Israel" implies that Absalom was attempting to convince the masses of his legitimacy to the throne.

In short, Absalom's actions were a blatant and calculated attempt to seize power, publicly humiliate his father, and assert his claim to the throne in a way that would be understood by everyone in Israel. The act was not merely about sexual gratification; it was a political statement with profound implications.