Radak: It is tents; he would enter with the Pilagshim to have Bi'ah with them. Yonasan translated Kinufin, i.e. one erects four poles, and puts curtains around them and on top.

Malbim (23): Achitofel said that he will not give his counsel until Avshalom has Bi'ah with them ¹ . Avshalom was 'forced' to obey, for Achitofel's counsel "was like one who asks Devar Hashem" (verse 23).