The creases of a model darker, as its author saw them in the modelling program. A glTF's occlusionTexture is read by itself, with its strength and either of its first two picture coordinates; in code it is occlusion on createMaterial or createMesh, the picture alone or { texture, strength, uv, wrap }.
It lowers only the light from all around, the ambient: the sun goes round, and a crease turned to it is lit like anywhere else, while the same crease in the shade is darker than the surface beside it. The left bottle keeps what its file brings, the right one has it taken away, and the crate in the middle has its creases painted in pixels.
Click the preview first so it hears the keyboard. Up and down change how much is applied.