It's a feature which allow you to set a huge quantity of nodes at the same time. It is the better way to create custom map generators because it can set a chunk of 80x80x80 nodes in less than 1 second. We currently call it the Voxelmanip.

How to get the voxelmanip ?

The standard way to get it is minetest.get_voxel_manip. Call this function without any argument, and you get the VoxelManip set on the most recently generated chunk, and the minpos and maxpos of this chunk. Mapgen Lib has implemented an other function, mapgen.get_voxel_manip, called with min_pos and max_pos, which returns a VoxelManip set on these positions, and a VoxelArea.