These are not the most htupid questions I've ever seen....

Mesh is the 'skin' that covers the skeleton of the sim world with all its humps, bumps, peaks, valleys etc

Landclass is what is on the surface of the mesh in specific locations - forest, woodland, open meadows, desert, cities, water surfaces etc

Bgls are compiled files that can be read by the sim program. They contain data relating to 2D and 3D static and dynamic objects which are located at precise latitude/longitude/altitude locations on the mesh. Bgls are teamed up with texture files (usually standard or extended bitmaps) which make the surfaces of the objects to which they relate visible in the sim.

Hope this helps!