Hey Fran, Which tutorial are you doing, the one on Eovia's site by Cajomi. If so the area you have circled changes the height of the Mountains where I think you want to raise the height of the infitie plane or water in the assembly room. When in this terrain editor window click the "hand" icon top right of GUI to take you to assebly window , select the infinite plane object and raise the position to .005. You are just trying to place the water at the correct elevation with respect to the terrain object. Good luck.