Topics get locked for some time (default is one hour) whenever a user edits, previews or saves a topic. This is to prevent simultaneous topic updates. There is a lock warning if another users tries to edit a topic while the lock is still on.
You can remove the lock by checking the "Release edit lock" checkbox in preview. This is a way to let other people know that you are done with your editing. Note: Please remember not to go back in your browser in case you checked the box and after that you decide to do more changes to the topic. Instead press the Edit link to re-edit your page.
Note: The "Cancel edit" link in edit mode lets you cancel out of an edit session without saving any changes. It also releases the edit lock, so that other people don't get the lock warning.
Note: The initial state of the checkbox can be set to on with the RELEASEEDITLOCKCHECKBOX preferences variable. See TWikiPreferences for more.