A search warrant is a kind of permission slip, signed by a judge, that allows the police to enter private property to look for particular items. It is addressed to the owner of the property, and tells the owner that a judge has decided that it is reasonably likely that certain contraband, or evidence of criminal activities, will be found in specified locations on the property.
As a general rule, the police are supposed to apply for a warrant before conducting a search of private property; any search that is conducted without a warrant is presumed to be unreasonable. This means that the police officers will later have to justify the search -- and why a warrant wasn't obtained first -- if the defendant challenges it in court.
A judge will issue a search warrant if the police provide enough information to show that:
The police usually provide information that is (1) based either on the officers' own observations, or (2) based on the secondhand observations of an informant.
If providing secondhand information, the police generally must convince the judge that it is "reliable." Usually, this means that the information is corroborated by police observation. For example, a citizen's tip that someone regularly delivers drugs to a certain location would be corroborated if an officer observes the person's routine.
However, corroboration of secondhand information is not necessary in every case. Sometimes a judge will issue a warrant if the source of the information is known to the police and has provided trustworthy information in the past.
To learn more about search warrants, see Search Warrants: What They Are and When They're Necessary.