The specific impulse (Isp) of a rocket nozzle is a measure of its efficiency in converting propellant mass into thrust. It is often expressed in units of seconds (s). Specific impulse is a critical parameter in rocketry because it helps in comparing and evaluating different rocket engines and nozzles. The higher the specific impulse, the more efficient the rocket is at producing thrust.
Keep in mind that specific impulse is often reported in seconds because it represents the time a rocket engine can produce thrust equal to its own weight if it were ejecting propellant vertically. Higher specific impulse values indicate more efficient rocket engines, which means they can achieve higher speeds or payloads with the same amount of propellant.