The RCWL-0515 is a little PCB that can be used as a motion sensor, similar in usage to passive infrared (PIR) sensors. This way you can make a light come on when you enter a room, or sound an alarm when the cat’s waiting at the door.

I bought a couple of these things online for €1.15 each, which makes you wonder quite how they can make something like this so cheaply. It contains one IC labelled “TT1712B”, one transistor labelled “32W” and a bunch of resistors and capacitors. The IC is some custom part for this purpose (I could find no information on it whatsoever), while the transistor is a BFR520^[1]https://www.nxp.com/part/BFR520#/, a 9 GHz NPN transistor originally made by NXP but in this case probably by some unknown copycat manufacturer (given that NXP discontinued it in 2016).