There are a few different technologies products use to detect motion. Our Motion Detector use "passive infrared sensors" to figure out when there is motion.
To understand how that works, we should first discuss how light works in general. Take a look at Figure 1 - this is a rainbow that we all know and love. What you might not know is that the rainbow also contains literally every color of light the human eye can process - red, orange, yellow, green, blue, indigo, violet, and everything in between, in that order. In the picture, "to the left" of violet and "right" of red, light doesn't suddenly cease to exist - human beings just can't see that light. Those bands of light are what we call "ultraviolet" and "infrared" light, respectively. There's also other types of light even further away from the "visible spectrum" that we can't see, that you've probably heard of - like microwaves or X-Rays.
So why does all that matter? Well, I'm sure you've noticed that when light bulbs consume energy, they also get warm and start glowing in the visible spectrum? In the same way, when humans and other animals consume energy, we also get warm and glow. But we burn much cooler than light bulbs, so the the light we constantly glow are invisible to humans - we actually glow in the "infrared" spectrum. See Figure 2 to see how we actually glow in the dark - I think it's super cool that we're constantly glowing!
Our Motion Detector has a tiny component inside creates a tiny electric current whenever infrared light hits it. This is because all light has energy, and can give that energy to other materials when it touches it. Think of how food in a microwave oven gets warm because of all the microwaves hitting it. Or, think of the sensor like a tiny solar panel,except instead of being sensitive to all of the sun's rays, it only looks at very specific light frequencies. The same way, the infrared light that we're constantly emitting will add some energy to the "passive infrared" sensor (PIR sensor) and the sensor will send a small electric signal, which is enough to for the Motion Detector to know something's going on.
The only reason this works is because walls, fans, and other common household items don't glow in the infrared region - only we do. Our pets also glow in the infrared range, so check out this guide on working with pets, and also read this guide on how to minimize "false alarms" with your motion detector.
Figure 1 - A rainbow, showing its true colors in order (red, orange, yellow, green, blue, indigo, violet.)
Figure 2 - A horse and a human glowing in the infrared spectrum. Fancy cameras let humans see what it might look like if we could see in this spectrum.