If you are looking for the manga content, deals with the aftermath of Kaede Domyoji discovering Tsukushi living in her home.
While on a trip, news breaks that Shizuka (Rui’s first love) is being courted by a French politician and may marry him. Rui is devastated and tells Tsukushi that "it's over". In a moment of vulnerability on the beach, Rui kisses Tsukushi, which is witnessed by Tsukasa (Domyoji). Hana Yori Dango Episode 21
Jan Di and Jae Kyung (Jun Pyo’s fiancée) have a heart-to-heart about their feelings for Jun Pyo. Jae Kyung is preparing to walk down the aisle, but tension peaks as it becomes clear the groom isn't ready. If you are looking for the manga content,
If you mean the individual chapter, it features Tsukasa and Tsukushi getting stuck in an elevator, forcing them to spend uncomfortable alone time together. In a moment of vulnerability on the beach,
Because Hana Yori Dango (Boys Over Flowers) has several famous adaptations, Episode 21 refers to different pivotal moments depending on the version you are watching.