To make use of the reflection cubemap, an object must have the Reflection Probes option enabled on its Mesh Renderer and also be using a shader that supports reflection probes. When the object passes within the volume set by the probe's Size and Probe Origin properties, the probe's cubemap will be applied to the object.

You can also manually set which reflection probe to use for a particular object using the settings on the object's Mesh Renderer. To do this, select one of the options for the Mesh Renderer's Reflection Probes property (Simple, Blend Probes or Blend Probes and Skybox) and drag the chosen probe onto its Anchor Override property.

See the Reflection Probes section in the manual for further details about principles and usage.