Protecting and unprotecting pages is very straightforward, but these operations require sysop permissions.
You can designate a page as a protected page by clicking the 'Protect page' tab, and supplying a comment (a brief textual description of why you are protecting the page).
There are several reasons why a particular page might be protected. On public wikis, a protected page is usually one which has been repeatedly targeted with vandalism, or where it is believed that any vandalism would have an unusually severe impact. On corporate wikis, a page may be protected when the content has been frozen via an approval process. However, there are also many good reasons for not protecting pages. As a sysop you must make this choice, and wield your power responsibly.