Reflection probes are basically like a camera in your scene that captures a spherical view of its surroundings and captures that view as an image. Its translated into a cubemap that reflective materials in the scene can then use to actually reflect. I’ll display what I mean.
I’ll go over this in URP since I’ve been handling URP situations up to this point. Just know that HDRP uses Refelction Probes a little differently and they go hand in hand with screen space reflections.
This is my scene without it:
To add it in, simply right click in the hierarchy, go to lighting, then select reflection probe. Its really up to you, but sometimes the reflection probe can make the scene look worse. Depending on your current lighting set up the reflection probe with act a little different. I currently have no directional light or skybox material in my scene. I did this in an effort to completely black out the scene so the only lights illuminating things are the emissions and spot, point, etc lights.
When I add the reflection probe and adjust the X, Y, and Z box size to fit my room, and bump up the intensity a little it looks like this:
Wildly different! The columns and the floor are definitely reflecting the green from the test tubes. You can also see that the glass on the test tubes has started to look a little more like glass.
You can adjust the reflection probe’s intensity, importance, size, etc to your liking. You can also have several reflection probes in a scene and tell the object sin the scene to use the cubemap from the nearest reflection probe, so you can get some really interesting results using them.
I definitely encourage checking out the Unity docs on reflection probes. Theres a lot you can do with them, and this article wont go into everything :)