API = Application Programming Interface.
This is the way other applications can communicate with Piwigo. This feature is also know as Web Services.
Examples:
This page lists all API methods available on your Piwigo installation, part of the Piwigo core or added by third-party plugins. For each method you can consult required and optional parameters, and even test them in direct live!
For more information you can consult our Wiki Piwigo Web API and our forums.