What is specific impulse and why is it important for rocket engines?

Specific impulse (Isp) measures rocket engine efficiency as thrust produced per unit weight of propellant consumed per second, expressed in seconds. Higher Isp means better fuel efficiency. Chemical rockets range from 250s (solid) to 450s (liquid hydrogen/oxygen). Isp depends on exhaust velocity: Isp = Ve/g0. It is critical because it determines the velocity change (delta-v) achievable for a given propellant mass fraction per the Tsiolkovsky rocket equation. Electric propulsion achieves much higher Isp (1000-5000s) but with very low thrust.