With Mirror anything you draw to one side of the drawing plane will be mirrored on the other side of the plane. This allows for creating perfect symmetry.
To access the Mirror tool click on the compass symbol in the top right corner. Once you click this a UI will appear at the bottom of your screen.
Select the Mirror button to activate the mirror. The button will turn purple which means you have your mirror plane turned on, so you should see another grey square appear in your drawing area.