By default, most Wikimedia pages can be edited by anyone, with or without an account. Some Wikimedia wikis are more restricted than others, but this is the standard approach. Wikimedia is based on the principle that anyone can edit.
However, protection does exist to prevent changing of certain pages or by certain people as necessary. For example, pages which have legal meaning on a project may be protected permanently to prevent errors being introduced. Some pages that are subject to vandalism may be fully protected to prevent changes by anyone except the most trusted users or partially protected to prevent unregistered or new user accounts from changing them. Pages that are fully protected can only be edited by users that have "sysop" or "administrator" rights or above. Pages that are partially protected are generally editable by logged-in users or by users who have had registered accounts that meet a certain participation threshold over a specific number of days.