If you have a file you don't need but you want to keep just in case, you can send it to your production's Archive. You can do any of the following to archive a file:
Click the pencil icon in the upper right corner to start selecting media and click the file's thumbnail, then click the archive icon
Click the ellipses next to the filename and select "Archive"
Open the file, click the ellipses in the upper right hand corner, and then click "Archive"
You'll still be able to access these files by clicking on "Archive" in your Media Library. If you decide to restore these files, you can follow the same methods above to select the file and click "Unarchive". They will retain any tags you attached prior to archiving, so you can still filter through your media.
If you no longer need a file and want to remove it permanently from your production, follow the same methods above to select a file and click "Delete". After confirming, the file will be permanently removed from the production. You cannot undo this action.