Unlike modern chemical rockets which burn liquid hydrogen and oxidizer to generate thrust, fusion drives use lasers to set off a self-sustaining hydrogen fusion reaction. The significant temperature contributes to exhaust velocity, resulting in extremely high engine efficiency. Without the need for oxidizer, more space can be dedicated to hydrogen. Additionally, the fact that thrust is generated at a constant rate (rather than in pulses, like thermonuclear pulse rockets) makes it far more maneuverable than its primitive atomic counterpart.
Fusion propulsion is the standard means by which modern vessels cross distances at STL velocities.