What is Eclipse doing when it says that it's updating indexes?

This is a general step that happens when m2e/m2eclipse (Maven integration for Eclipse) is installed, whether projects are actively using it or not.

This step can be disabled through the Eclipse preferences: Window / Preferences / Maven / "Download repository index updates on startup". This option is on the main "Maven" preference page (not a child page). Just uncheck the box to prevent this from happening.

The file that this is downloading is an index of all the available dependencies available in the Maven central repository for use in Maven-enabled projects, allowing them to be easily chosen and searched against within the Eclipse UI. It is mainly a user convenience, and isn't mandatory.