Reflection Probes

Reflection Probes are objects that grab a view of the world around it and applies what it sees to any objects within it's boundaries that are shiny enough to be reflective. This can add a great deal of immersion to a scene so that objects feel grounded within their surroundings.

When building scenes with static lighting Baked probes are the best option. Their surroundings are baked and exported with the scene and operate under the assumption that drastic lighting or environment changes won't happen in a scene. They are also the cheapest type of reflection probes because they don't need to update at runtime.

Realtime reflection probes are best used for environments with changing lighting, like a day-night cycle. Realtime reflection probes update in Curio at a rate of once per minute, and must be withing a game object called Reflections_GRP, under your _World object.

Without a reflection probe shiny objects will reflect the skybox by default, and if you have an interior scene this will look very wrong.

Official Unity Documentation for Reflection Probes.