Select the button to reverse the last action. It is possible to undo commands back to the start of the editing session of the drawing. Commands that can be undone include any that affect the appearance of an entity i.e. when creating, modifying, deleting or changing the properties. Commands excluded are settings and changes to the current zoom.
To reverse an undo action use Redo. Any entity creation or modification empties the Redo list, therefore care must be taken when executing multiple undo's as you will not be able redo some actions if any entity is created or altered. For example you could draw say fifty lines, if you then undo ten actions, the last ten lines will be undone. If you were then to modify any one of the remaining lines the Redo list will be emptied and the ten lines 'undone' will be lost.