You reshape the terrain to your will. You can alter the terrain of a 50 foot cube centred on a point you can see within range. You can subtract or reshape the present terrain as you choose within the cube, but you can only add material that is present within the cube when you first cast the spell.

The spell takes the entire hour that you spend concentrating on it to change into the shape that you chose when you first cast the spell. If you break concentration part way through, the change halts and the terrain will only be partly shifted depending on how long the spell was concentrated on for. The DM will decide what this looks like.