When a page is displayed for editing, a lock is put in place to prevent other users editing that page. The lock stays in place as long as the user keeps the page open; it is removed when the user browses to another page, or closes the browser tab or browser. Locks are removed automatically if the page has not been updated for some time.
Edit locks can be removed manually by selecting Maintenance from the System drop-down, and choosing Clear edit locks. This may be useful if, for example, a user was editing a page and their browser or computer crashed or they lost internet connection.