This feature is available when you click the Edit button within the option box on the opportunity page, then click the Content tab in the top left corner of the window that opens. A floor plan must be associated with a product for this feature to work, see this article for more information (linked to Editing Product Content article).
Click Draw on Floor Plan. You will click to plot individual, connecting points on the floor plan - similar to drawing a custom shape in Microsoft Office. Once the area or areas that you would like highlighted are complete, you are able to preview if you’d like by clicking the Preview button. Once happy with the highlighted areas, click Save in the top right corner of the Floor Plan box, then Save again at the bottom of the window.