Active scanning technology is things that emit some form of radiation in order to scan things. This could be radio waves (in which case the technology is called radar), laser light (which is lidar), infrared waves (which is how the Microsoft Kinect works), x-rays (x-ray imaging and CT scanning), sound waves (sonar, or ultrasound imaging), or even seismic waves penetrating the Earth, which can be produced by controlled explosions (seismic tomography).

Active scanning is good, because you can detect things which might not otherwise be visible or detectable in other ways. For example, radar can detect planes flying at night or in thick cloud. The problem with active scanning is that your scanner is emitting radiation, which makes it highly visible to anyone with the means of detecting that radiation. If you can detect radio waves of the appropriate frequency, then a radar installation will glow brightly and probably be the most obvious thing anywhere near you.

In a game example, you might want to scan for enemy ships. They are hard to see, but if you use radar you could easily find them. The trouble is, using radar gives your own position away and makes you highly visible.

The alternative to active scanning is passive scanning. In passive scanning, you observe radiation that is being emitted or reflected by other objects, but you don't send any out yourself to cause those reflections. One ubiquitous example of passive scanning is human eyesight. You detect objects near you by observing the light reflected off them or being emitted by them - but your eyes don't emit any light to illuminate them.*

Passive scanning is good because you don't give away your own position. All you are doing is collecting information that is already out there. On the other hand, it's not so useful in some fairly common situations, such as visual scanning in dense fog, or darkness.

Bringing this back to gaming, if your game involves appropriate technologies, you can pose the question to your players. Do they want to scan actively and be sure to detect everything, at the risk of giving themselves away, or do they want to scan passively, not giving themselves away, but at the risk of not detecting nearby danger. It's an intriguing question, which leads to many tactical roleplaying opportunities.

* There used to be an ancient theory that human sight did work by emitting light.

Transcript

Luke: I guess we hit some kind of park. That's lucky. We'll conceal the ship here, walk to the nearest building, and start looking for Yoda.
R2-D2: This might be like Tatooine. Maybe he's hiding out in the wilderness.
[SFX]: < fip-jewhree bikali-taz-ni-bebedooby bap-jing dooboot pading-dooby >
Luke: Even better.
R2-D2: Scanning for life forms.
[SFX]: < bop ding >
Luke: Passive scans only, remember.
GM: Life forms everywhere. All around you. Too many to count.
R2-D2: Whoa! Okay, electromagnetic activity.
[SFX]: < eefloo gidip >
GM: None. None whatsoever.
{beat - Luke looks around}
{beat - Luke turns to face R2}
Luke: R2, I've a feeling we're not on Tatooine any more.