Microsof's official doc says:

If you want to make changes to a file on a SharePoint site and want to make sure no one else can edit it, check out the file. When you have the file checked out, you can edit it online or offline, and save it—multiple times, if necessary.

It's a form of lock on files.
Check out to lock a file.
Check in to unlock it.

Resources that solved my question:
- https://support.microsoft.com/en-us/office/check-out-check-in-or-discard-changes-to-files-in-a-sharepoint-library-7e2c12a9-a874-4393-9511-1378a700f6de

- https://medium.com/jumpto365/the-basics-of-sharepoint-check-in-and-check-out-e7db1c63d860