Under the Fourth Amendment, Americans have the right to avoid improper searches and seizures of property. In order for a search warrant to be issued, there must be probable cause.
Probable cause can be tricky. It is a thin line and easily crossed, so probable cause is easily abused. In order for a judge to grant a warrant based on probable cause, there must a reasonable basis for believing a crime has been committed by a person to be arrested or that evidence of a crime exists at the place to be searched.
reasonable basis for believing that a crime may have been committed (for arrest) and that evidence of the crime is present in the place to be searched (for search). (Cornell University Law School)