Sysops can protect pages to stop them being edited. The most common reason is to stop an edit war or stop vandalism.
A list of pages which are protected, and reasons why they are, is found at Wikibooks:Protected pages.
Do not let a page stay protected for too long. Unprotect it after some time in order to allow editing.
See: Protected pages considered harmful.
Semi-protection of a page stops unregistered editors (I.P. address) and editors with accounts newer than four days from editing that page.
Semi-protection should only be considered if it is the only option left available to solve the problem of vandalism of the page. It is to be used in response to serious and repeated vandalism, such as when certain pages suddenly become high profile due to current events, are being linked from a high-traffic website, or being affected by evasive vandals. It not as a pre-emptive measure against the threat or probability of vandalism, but only when there is evidence of a serious problem.