Give Me thy heart! adore Me! serve Me! cling
The grand synthesis: Krishna has taught karma, bhakti, and jnana. Before his final, best-known command — to abandon all duties and take refuge in Him alone (18.66) — he first asks for something more intimate: Arjuna’s whole heart, given in faith and love and reverence.
Krishna’s most personal appeal for devotion — fix your heart on Me, love Me, worship Me — spoken just before his ultimate imperative in the next verse (18.66, the charama shloka).