Seam stitching is a feature that smooths unwanted hard edges in GameObjects rendered with baked lightmaps generated by the Progressive Lightmapper.
When Unity bakes lightmaps, it identifies Mesh faces that are close together but separate from each other as being separate in lightmap space; the edges of these Meshes are called “seams”. Seams are ideally invisible, but they can sometimes appear to have hard edges depending on the light. This is because the GPU cannot blend texel values between charts that are separated in the lightmap.
Seam stitching fixes these issues. When you enable seam stitching, Unity does extra computations to amend the lightmap to improve each seam’s appearance. Stitching is not perfect, but it often improves the final result substantially. Seam stitching takes extra time during baking due to extra calculations Unity makes, so Unity disables it by default.
Seam stitching works with the Progressive Lightmapper. Seam stitching only works on single GameObjects; multiple GameObjects cannot be smoothly stitched together.
You can enable seam stitching on any GameObject with a MeshRenderer component, like this: