There are three types of Light Probes
- Reflection Cube Map
- Reflection Plane
- Irradiance Volume
The two first are, as the name suggests, to generate reflections for materials. They may seem useless at first because you can enable Screen Space Reflections in EEVEE render panel, however screen space reflections have limitations. They are very fast to generate, but can only capture objects that are currently displayed in the viewport, that is inside the view fustrum.
These probes allow calculation more complex and accurate types of reflection through use of a helper object (the probe).
As the name suggests Reflection Plane are the most simple ones, suitable for mostly flat surfaces like mirrors, glass panes, floor reflections, or rainy pavements. On more complex objects they will probably generate weird results.
It has a clipping distance that will limit which objects are visible in its reflections. They are relatively cheap in terms of computation and don't require baking.
Objects with reflective materials that reside withing its range (the bounding box around it) will be influenced by what this plane "sees", benefiting from the the probe reflection map.
Reflection Cube Map
Similar to the plane this will generate a more complex reflection map suited for curving shapes. Unlike the plane though this is not a real time process and requires baking, specifically the Bake Cubemap Only option from the Render Panel in the Properties Editor.
It also has clipping distances which affect the range of what is reflected and what gets clipped away. Moving a reflective object away from its influence radius also removes it from it's "effect".
Irradiance Volume is a different kind of probe, it calculates indirect lighting and shadows rather than reflections. Real time rasterization engines like EEVEE although very advanced can't really calculate indirect lighting by themselves, thus need a help generating and displaying this information.
Irradiance Volume preform both these tasks by storing the indirect lighting information in a grid array of points, the resolution of which can be adjusted from its Resolution XYZ in Object Data Properties.
Once again Clipping affects which objects are in range for calculation, and everything inside its volume gets influenced by the light calculation.
Likewise, since this is potentially heavy calculation it requires baking from the render panel to show it's influence. The higher the XYZ resolution the denser the grid, the heavier the calculation thus slower process, with the resulting higher quality effects.
- Add probe object
- Adjust position
- Scale it so it encompasses all desired parts of the scene
- Adjust clipping distances and grid density as desired
- Bake (from render panel for Cube Maps and Irrandiance Volumes)
These calculations should be stored within the Blend file and available next time you load it, so you aren't required to bake again.
Here are the results of the various bake steps
Cube Maps only (no indirect lighting, notice the reflections)
Bake indirect lighting (notice the color bleeding, softer lighting, and proximity shadows)
You may use several probes per scene to get denser details where required, but each one consumes memory and has a performance penalty on the viewport, so use them wisely.