Because Drupal gives the ability for multiple users to work on the same Web site, the situation can arise where more than one person can try to edit the same page. Thankfully, Drupal includes the checkout module to help with this. Whenever someone goes to edit the page, the checkout module will automatically checkout that page and keep anyone else from being able to make any edits. By default, a checkout will last 15 minutes; after that, the checkout will expire and others will be able to edit it again. Should you take longer than those 15 minutes to edit the page, then your changes will be saved as a draft. Please see the Draft tutorial for more information on drafts.
Should you try to edit a page that is checked-out, you will recieve a notice saying such, along with who has it checked out, when it was checked-out and how much time is remaining before the page can be edited again.
You can see which pages you have checked-out from your My account link.