Files can also be checked out by clicking on the ellipses  to the right of the file name. In the call out card click on the second ellipses  and click Check Out. 

When a file is checked out, a green arrow will appear on the file . Also the Checked Out To column will let you know who the file is  checked out to. If you can not see this column it may not be included in the current view.

Checked out files can only be updated by the person who has checked them out until they check them in. This will allow others to edit the file.

Other users can view the file however, they can not see other users changes made to a file until it is checked in.