I heard the Source 2 editor has (relatively offline, think blender viewport style) ray tracing as an option, even though no games with it support any sort of real time RT. Just so artists can estimate what the light bake will look like without actually having to wait for it.

So what people are talking about there is lightmaps, essentially a whole other texture on top of everything else that holds diffuse lighting information. It's 'baked' in a lengthy process of ray tracing that can take seconds to hours to days depending on how fast the baking system is and how hard the level is to light. This just puts that raytraced lighting information directly into a texture so it can be read in fractions of a millisecond like any other texture. It's great for performance, but can't be quickly previewed, can't show the influence of moving objects, and technically can't be applied to any surface with a roughness other than full (so most diffuse objects but basically no metallic objects, those use light probes and bent normals usually, and sometimes take lightmap information although that isn't technically correct and can produce weird results in some cases)

The solution to lighting dynamic objects in a scene with lightmaps is through a grid of pre baked light probes. These give lighting to dynamic objects but don't receive it from them.

Honestly, this wouldn't have been an issue, ever, if we wouldn't have switched to "release fast, fuck quality, crunch ya' plebs!" It's yet another solution for a self-generated problem.