If you want your scene's environment to be reflected by game objects, featuring reflective materials (such as the ones with high Metallic or Specular levels), then you can achieve such effect using Reflection Probes. They allow for real-time, baked, or even custom reflections through the use of Cubemaps.
Real-time reflections can be expensive in terms of processing; in which case, you should favor baked reflections, unless it's really necessary to display dynamic objects being reflected (mirror-like objects, for instance). Still, there are some ways real-time reflections can be optimized. In this recipe, we will test three different configurations for reflection probes: