If this property is enabled, the BasePart|part will not contribute to the total mass or inertia of its rigid body as long as it is Weld|welded to another part that has mass.

If the part is its own root part according to BasePart/GetRootPart then this will be ignored for that part, and it will still contribute mass and inertia to its rigid body like a normal part. Parts that are massless should never become an assembly root part unless all other parts in the assembly are also massless.

This might be useful for things like optional accessories on vehicles that you don’t want to affect the handling of the car or a massless render mesh welded to a simpler collision mesh. For instance, to create a massless MeshPart, you would use the follow code:

local mesh = Instance.new("MeshPart")
mesh.Parent = game.Workspace
mesh.MeshId = "insert meshId here"
mesh.Massless = true

See also

• article/understanding root parts|Understanding Root Parts, an article documenting what root parts are and how to use them