Our standard desktops are sold blank, with no holes drilled in. You can still assemble this way and it's not very difficult, but you do need to pierce the hard top surface of the desk-top.
Pre-drilling is when we drill small pilot holes in the underneath of the desktop to penetrate the hard surface. You can then screw easily into the softer core of the top to attach the desktop to the frame.
Metal inserts are metal components fitted into the underneath of the desktop. You attach the desktops to the frame by tightening screws into the inserts with a hex key. This removes the need for a screwdriver and means the top does not get punctured during assembly. It also means you can remove the top as many times as you need.