Markers are accessible from the side menu (click the -icon on the upper left screen of your browser window to open the menu).
This is a location where shapes you have marked appear as a quick reference. By clicking on one of your markers, the screen centers on the related shape.
To mark a shape, simply right click it and select the 'Set marker' option. Once marked, you can remove the mark either from the markers menu or by right clicking the shape and selecting 'Remove marker'.