This post shows how Planet Foundations off-canvas sidebars work using features from Zurb’s Foundation. The sidebars are probably visible if you are on a desktop or laptop. Now re-size your browser to the size of a tablet or smart phone. Note the accordion style buttons on the right and left. Click on these and watch the sidebars slide in.
If you want the sidebars to slide in on large screens, go to Appearance-Customize, Sidebars and enable Sidebars on Large