Check In and Check Out
Important document and page libraries usually require the use of check in and check out to prevent accidental changes to items and prevent conflicts when multiple people are editing the same content. When this feature is enabled, you must check out an item before you can change it. Once the item is checked out, no other users can edit the content until the author either checks the item in or an administrator undoes the check out.
Whenever a user checks an item in, a new version is created based on the configuration of major and minor item versioning.