They're invisible light bulbs.
Wait, did I just blow your mind?
When you see a security camera like the one above, there is often a ring of what appear to be light-emitting diodes around it, but they never seem to light up. Or do they? If you've ever wondered how a security camera could see so well in low-light or no-light situations, there's your answer. These LEDs give off infrared light, which you can't see but your security camera can.
You've seen infrared LEDs plenty of times before, although you probably didn't realize it. They're used in pretty much every remote control to send commands to the device you want to control. They blink in predictable patterns and a sensor on the device sees those patterns and reacts. In a security camera, they cast a large beam to the area just in front of the camera in order to help the camera's low light performance.
By the way, people sometimes ask if infrared LEDs give off heat. The answer is that they don't give off any more heat than any other LED. People confuse infrared radiation with heat because they are so often part of the same process. One way that heat is generated is when infrared radiation excites something in its path. Infrared is specifically good at exciting things in its path, much better than visible light, so heat is a very common effect of infrared. You don't need to worry about that with infrared LEDs though because the amount of radiation produces is nowhere near enough to heat anything up to any measurable degree.