To create proper reflections in your objects an environment map is needed. The map is what you will see in the reflection of the object. This image will be used spherical to wrap around the whole scene. For the map, you can use a .jpg in 1024×1024 resolution. The intensity will change the shininess of the object.
If you use the material MeshPhysicalMaterialExt you change the intensity in the PBR tab. You can find the explanation of this later on in this document.
SSRL stands for screen space reflections level. This controls the amount of other object reflecting in this material. It is commonly used to create more subtle reflections such as on wet floor surfaces or in puddles.