The Restore Lines command restores a block of edited lines back to their original contents. The lines will be as they were when you first opened the file.
Restoring lines will lose those changes you made since it was first opened, even if you saved the file. The file that is saved on disk is not altered. Only the lines in the open file buffer are restored.
The Restore Line command is undo-able. This gives you a powerful, out-of-order undo capability.
You can access the Restore Lines command quickly by right-clicking in the selection bar (left margin) area next to a block of modified lines and choosing it from the right-click menu. You can see what lines are modified by turning on line revision marks. (See Preferences: Display command.)