The high-quality scans, provided by your lab, take up a great deal of space on the server. The lab may have a policy on how long your files may remain.
The checked roll icons indicate rolls that were completely downloaded, in this or a previous session. Selecting these icons will show the download date. Since you may use multiple computers, the name of the machine, that was used to download, will also be shown.
Once you have confirmed that the downloads are complete and secure, the rolls should be deleted from the lab's server, to reclaim space. This will save the lab time in administration.
The deletion buttons in the toolbar work similarly to the ones for downloading. Note that deletion may only be done on entire rolls, not individual files. The deletion functions are locked out while any downloads are in progress.