If you turn a light bulb on for a few minutes it gives off light, and it gets hot. When you turn off the lightbulb, there is no more light, but the lightbulb is still hot. Likewise, the light from the sun heats the oceans. Even after the sun sets, the oceans can still give off the heat. As they orbit the Earth, active instruments send pulses of energy to the Earth to take measurements.
Passive instruments measure the energy which is given off by the Earth.
As it orbits the Earth, it simply measures the radiation which the Earth gives off. It sends pulses to the Earth to measure the height of the water.