Pinning sketches allow you to highlight your favorite sketches to the top of your profile on OpenProcessing, so that people visiting your profile can easily find those. To pin a sketch, click on the three dots then "Pin" icon on a sketch on your profile.
Currently, there is no limit to the number of sketches you can pin. Each time you pin a sketch, it will be moved to the top of your profile. You can unpin by clicking on the same icon.