The mesh is the shape of the terrain.
The landclass is the type of terrain, defining the position of the forests, cultivated fields, cities, villages, roads, lakes, coastlines, etc... The landclass defines which texture goes where.
A scenery is a bundle, that can contain a mesh, a landclass, and some specific objects(buildings, statics planes boats etc..., local ground texture for photoscenery, etc...).
It's generally recommended to place the meshes at the lowest part of the scenery library, below any additionnal scenery.