Probable cause is reasonable suspicion that someone could potentially be a criminal, or is suspected they are going to commit a crime.
Example 1: The police come to Dave’s house, and they’re looking for a graffiti suspect. There’s nothing suspicious going on, and he even has an alibi for when the graffiti artist struck. If they search his house without his consent, that is violation to the Fourth Amendment.
Example 2: Rose is in the same situation, however, spray cans are around her house and she’s acting funny. Things aren’t lining up. This would constitute as probable cause because they have reasonable suspicion that she’s done something and therefore would not violate the Fourth Amendment.