You can designate a page as a protected page by clicking the 'Protect page' tab ('protect' in the drop-down menu in Vector skin), 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. For instance, it may hinder useful contributions by impacted users. As a sysop you must make this choice, and use your privileges appropriately.