A passive radar is a radar system which receives only, instead of alternating transmission and reception. These systems are used in a variety of settings, and they have a number of benefits which make them targets of interest for many military technology developers. Essentially, a passive radar is a very finely tuned pair of ears, sensitive to high frequency radio waves rather than sounds which can be heard by the human ear.
Many people are familiar with the way in which radar works: A signal is transmitted, and a receiver waits for the signal to return, drawing inferences from the returned signal about what kind of objects the signal interacted with, and how far away they are. For example, a ship could use radar to look for potential enemy ships in the area. Unlike sonar, which uses sound, radar uses high frequency radio waves.
Passive radar has the same ability to pick up microwave energy, but it does not transmit it. Instead, is uses reflections from other objects and the original source to gather information about targets, working passively rather than actively to identify objects in its vicinity. Using this information, the device can provide information about range, velocity, and location which can be used to make decisions. Passive radar on an aircraft, for instance, might be used to identify other aircraft in the area for the purpose of avoiding them.
From a military perspective, the clear advantage to passive radar is that it is stealthy. Since it does not transmit, it can be difficult for an enemy to locate, because there is no signal to track. Instead, passive radar simply sits and waits for signals of interest. Even if a passive radar array is identified, it can be difficult to jam, because there isn't a way to know which frequencies it is using, although tactics may be used to spoof it for the purpose of confusing it. Using passive covert radar can improve mission safety and allow people to have access to radar data continuously.
Passive radar detection of targets of interest is also less costly and complex than working with an active radar system. This makes it less expensive to implement, and can make it appealing in settings where radar needs to get operational quickly. Many different things can be used as a passive radar reflector, allowing the technology to utilize a variety of objects in its vicinity to collect information.