In Star Trek: Enterprise, the NX-01 is equipped with an experimental Warp 5 engine.
The Warp 5 engine is the major propulsion achievement that makes Enterprise's deep-space mission possible. Before this development, human warp ships were considerably slower and could not explore distant regions of space within practical mission times. Zefram Cochrane's earlier warp breakthrough provides the foundation, while the NX-01 represents the next major human advance.
The engine was developed through the work of Henry Archer and his team, including his son Jonathan Archer, who later commands Enterprise. The ship's ability to reach Warp 5 gives humanity a much greater role in interstellar exploration.
The phrase does not mean Enterprise travels at one permanently fixed speed. Warp speed is a performance scale, and the ship's top capability changes with conditions and later upgrades. Warp 5 is the engine's defining design target and historical significance.