What's a lightmap?

Just as a model might have a basic albedo/diffuse map, a normal map, and etc., a light map is a texture applied to the surface of that model. Using Bakery or whatever, we pre-calculate the lighting using higher quality lighting calculations than we can afford in the actual scene, and then save it into a texture.

This means that your lightmapped models need a seperate set of UVs designed to hold the lightmap. These can be automatically generated in Unity by enabling "Generate Lightmap UVs" on the model file, or created in your modelling tool as a second UV channel. Blender can take an existing UV set and re-unwrap it as a lightmap UV to make this process easier.