An active sensor/component requires an electrical power source to work. A passive one does not.
Ultrasonic sensors, radar, and video cameras are active. A strain gauge (whose resistance changes with pressure applied) is passive. Some types microphones are active while others (like piezoelectric microphones) are passive. THey generate an output with only the input energy they are sensing. They do not need a power source to generate an output (though the output will usually be very low and need to be amplified with an active device).
Thermocouples and thermopiles are passive. THey generate a voltage output using the temperature energy they are sensing. They do not require a battery to make their output. But it is very weak and probably needs to be amplified with an active device like an op-amp.
Some RFID tags are passive, others are active. The active ones need a battery present to work. THe passive ones take the RF energy being transmitted by the base unit, modify it somehow, and reflect it back out to be received by the base station, but they have a much shorter range than the active ones.