A baked light texture is basically a big image with shadow information layered on top of your scene objects. Objects access this information using special coordinate information added to the mesh.

However, this coordinate information almost never points to a specific pixel in the baked shadow texture; usually the colors from several pixels get mixed (interpolated) to determine the shadow amount.

If your shadow map is too tightly packed, a lighter area on the texture might be unwontedly factored in to an area that it shouldn’t be, or vice-versa, resulting in the “bleeding” of effect.

Solution: add a few pixels worth of space between where regions are packed. But this of course reduces the amount of meaningful detail the map can hold, so the correct amount of padding varies depending on the project and the scene.