Specific Impulse (Isp) is measured in seconds and can be described by this equation:

Isp = F/(m_dot * g_0)

Where F is in Newtons, m_dot is the mass flow of propellant in kg/sec, and g_0 is the gravity of Earth at sea level in m/s^2.

One fun way of viewing it is that an Isp of 465 seconds means that 1 kg of propellant can provide 1.0197 kg (10 Newtons) of thrust for 465 seconds.

With the above example, it is easier to see why Isp is used to measure the efficiency of a rocket engine, similar to how miles per gallon is used to measure a car’s efficiency.