The checkout feature prevents multiple contributors from editing a file at once. Admin or read+write permissions are required to check out files.
Check out a file
Open your project, then click the Files tab in the navigation bar.
The "Files" page will appear.
Click next to the file you want to check out, then click the Check out file button in the toolbar.
A modal will appear.
Click the Check out file button.
The file will be checked out to you.