The surface level setting, located in the bottom left corner of the Voxel Editor viewport, allows you to define the height of a tile's surface or ground. This height is used by the Map Editor and the Add Tile scripting function when placing the tile into a map and provides a way to vertically align a set of related tiles more easily.
For example, the dungeon entrance tile shown above is having its surface level set to the height of the grass at the top of the stairs. Normally you would need to adjust the grid downward when placing the dungeon entrance into a map so its height matches up with the grass, but now the tile itself will be automatically adjusted downward so the top aligns with the adjacent grass tiles. This helps minimize the need for manual grid adjustments when placing tiles that need to have their surfaces aligned.