The table shows the list of Maven repositories, encountered in the current project (including repositories in settings.xml), with their URLs, type (local or remote) and the date of the most recent update.
Indexed Maven Repositories
This area contains Maven repositories that are configured in the Effective POM file which lists the default configurations, profiles and goals. IntelliJ IDEA updates the list of repositories automatically.
If you open a project that contains additional repositories specified, then the repositories are added to the Indexed Maven Repositories list and you can update the indexes.
Click this button to update indexes of the selected repository. It might be helpful in case you expect to get information for newly deployed artifacts such as new versions of libraries that you use in the project. Also, when you use Maven dependencies completion in pom.xml or generation of Maven dependencies using Maven Artifact Search dialog.