How checkout works.
How does checking out a file for use in Word 2007, Excel 2007, or PowerPoint 2007 work? Here's an overall explanation.
Check out the file.
At this point, the file is reserved for only you to work on.
Edit and save.
You edit the file by clicking its file name in the library. When the file opens, you work on it and save as many times as you need to. However, when you save, your changes will not be seen by others yet.
Check in the file.
When you check in the file, others will see your changes and will be able to edit the file.