The bed mesh is a probed map of the surface of the bed plate, normally automated and with a z probe, that describes how flat (not how level) the surface of the bed is above the level plane. It's a map of the hills and valleys decribing the warpage of build surface.

It's often erroneously called “ABL”, “Automatic Bed Leveling”, yet has nothing to do with the bed being level.

The Klipper Documentation for the bed mesh.

All3DP: Klipper: Bed Mesh – Simply Explained