When you check out the current working version of a file, it becomes unavailable to other users. They can still view the file, but they cannot make any changes until you check the file back in. When you check in the revised document, users can view the new version and check it out for additional revision.
To check out a file:
If you do not already have the current version of the file on your computer, click Download the current version now.
After making your changes to the file, check in the revised file. If you decide not to make any changes to the file, click Undo Checkout.
The project stores copies of all versions of the document and a complete revision history.