It's the Auto Analyzer and it's analyzing the videos and photos that you have added to the Organizer. Ideally, you want to just let it do it's thing until it's worked its way thrugh all your media. Once analysis is complete, then a number of new automated features can be used that rely on the analysis.
If you really want to turn it off, you can go to the Organizer Preferences and turn it off.
Elements Auto-Analyzer is a process that starts when you launch Elements Organizer, or when you right click some clips in PrE and click Run Auto Analyzer, or when you aplly an Instant Movie with Auto-Analyzer selected.
This process is responsible for analyzing the videos and photos in your current catalog, and finding the nature of your media, i.e. face detection, quality detection, bright/high contrast, etc.
Once your media is analyzed the auto- analyzer applies tags to your media about what it has found, these tags can be viewed by right clicking your media in Organizer and selecting Remove keyword tag.
You can manually kill the Auto-Analyzer process from the Task Manager/Activity Monitor or stop it from starting from Organizer Preferences as told by Bob.