On occasions a user may decide they no longer intend to make any changes to a file they had checked out.   The Undo facility exists to let the user roll the checkout back enabling other users to checkout the file instead.

Also a file they checked out may have been lost or they may be unavailable to check the file back in and the Project manager or system administrator may decide they are prepared to lose any changes that might have been made to that version of the file and cancel or 'undo' a checkout.

Any user can undo their own checkout but only a user with admin permissions can undo a different user's checkout.  

If they open the File Explorer and click on the file in the tree view then the status will give the user the file is checked out to. Whereas for other users the 'in' and 'undo' buttons are grayed out, if it is your checkout or you are an admin account user, the undo button will show 'lit' and can be used to cancel the last check out.  Thus effectively resetting the file to its previous version.

Also the user can review their own checked out files on the checked-out files form.  This will only show the user the files checked out to them (so cannot be used by the admin user to undo someone else's check out) to undo a file here, simply select the tick box next to the file and press the undo button at the bottom of the form. By using the All Projects tick box users can see all files checked out to them irrespective of which project although this all projects view is just viewonly and multiple undos cannot be underteken across multiple projects.