Sela is a large rock, cliff face, or crag — a massive outcropping of stone used as a natural fortress or refuge. It differs from tsur (another rock word) in that sela tends to emphasize vertical, imposing rock faces — cliffs and crags. It can also be a proper name for a city (Petra) or the city of Edom.
David famously fled to the sela in the wilderness, both physically and spiritually. Psalm 18:2 and similar passages use rocky imagery to describe God's character — immovable, towering, enduring. The cliff that provides physical refuge becomes a symbol of the God who cannot be moved, eroded, or overcome. To call God 'my rock' is to declare His absolute reliability. In the NT, Christ is the rock (petra) upon which the church is built — the same immovable foundation.