Layers are aspects or facets of a Plane, so to speak, different ways the Outer Planes express their themes. Each layer is infinite and contains its own distinct environment, yet intimately connected with other layers of the same plane (through Portals, planar+conduits, or more abstract connections), sharing the larger plane's alignment and base nature. Each layer is an infinite space, at least conceptually.
Arborea's three layers - Olympus, Ossa, and Pelion - represent three ways of viewing Chaotic Good (some say they represent present sensations, memories of sensations, and forgotten sensations).
Bytopia's two layers - Dothion and Shurrock - represent two ways of viewing law-tinged Good (some say they represent domesticated and undomesticated goodness).
There are a variety of different metaphysical ways layers can be arranged in relation to one another. The different layers of the Abyss grow chaotically from the plane's entrance layer, the Plain of Infinite Portals. The glooms of the Gray Waste meet at a single point, the Town at the Center. The layers of Arcadia are joined by high mountains. The layers of Baator exist "beneath" one another like a sloping pit, while the layers of Mount Celestia exist "above" one another like a rising mountain. The layers of Bytopia face one another, so each is visible in the other's "sky." The third layer of Ysgard, Nidavellir, is "underground" in relation to the other two layers of the plane. The layers of Carceri exist within one another like nesting dolls. The furnaces of Gehenna are visible as distant mountains floating in the void.
Only Outer Planes are known to have layers as such, and not all Outer Planes have them. Mechanus and the Outlands lack known layers (although the Outlands has its rings).
Limbo has planar+conduits connecting different regions of its indistinguishable chaoscape as if these were different layers, but all of Limbo's "layers" appear more or less the same (or equally varied), and the same features have been found in all known "layers" of the plane, so if Limbo does have layers they're mostly irrelevant.
A layer may be divided into realms, though there are parts of every layer where divine realms do not impinge.
Sources
Planar Handbook
Planes of Law
Planes of Conflict
Planes of Chaos
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