I believe some of these would be files kept in the task view for reports that were run, other may be from historical data sets and so forth. I wouldn't just delete it though, there is a report you can run to help you determine what some of that data is. I know for my area having items stay in the Task Status view can chew up a lot of space as that window gets full for each user. Plus when some users close a report the click on Yes to save it thinking it will keep it in the task status view when it saves it back to the historical dataset .
Here is a query you can run to look at some of this data
When starting in the Select Data Source window, click on the Show Advanced Data Sources
Two new sections will show up (Bindview ActiveAdmin Session Logs and Bindview Internal Data Sources)
Expand Bindview Internal Data Source
Select the Historical Datasets field.
That will allow you to run some reports on the items that are taking up space. I believe you can use active admin inside of that area to clean up some data.