During the creation of a route, it can happen that you accidentally click somewhere or do something else so that your route is no longer as you want. You want to be able to undo this easily. The undo button is exactly what you need.
When you open a new route, both buttons will be grey and you will not be able to click on them. If you then place a waypoint but it is not right, you can cancel the action (placing the route point) by clicking button number 2. If the point was correct, you can press button number 1 to place the waypoint back again. This way you can easily set a number of points and still go back to the original.
Suppose that you undo a number of steps and then place a new point, you can not get back the unused steps. This is the same logic as you might know from Microsoft Word and other text editors. You can undo up to ten steps.
Incidentally, you can also perform the Redo operation (button 1) by clicking on "Control + y" or "Control + shift + z". For the Undo operation, this is "Control + z". Users who have a MacBook should use the "Command" button instead of the "Control" button.