You can access the Weebly App Center from within your website builder in the top navigation menu item called "Apps".
You can install, purchase, and use any app you'd like. These are additional tools you can use to augment the functionality of your website. Similar to WordPress Plugins, apps are built by individual developers (not by Weebly or Flipcause unless specifically noted). This means that payment and technical support are handled directly with the developer of each app.