A more thorough explanation:

The plain-sight rule is a legal principle that allows police officers to seize and use as evidence an item that is in plain view during a lawful search or from a lawful position, without a warrant, if the officer has probable cause to believe that the item is evidence of a crime.

For example, if a police officer is conducting a search of a suspect's home with a warrant and sees drugs on the kitchen counter, the officer can seize the drugs and use them as evidence in court. Similarly, if an officer is on patrol and sees a gun on the front seat of a car during a traffic stop, the officer can seize the gun and use it as evidence if there is probable cause to believe that the gun was used in a crime.

The plain-sight rule is based on the idea that if an officer has a lawful reason to be in a certain place, they should be able to use their senses to detect evidence of a crime that is in plain view. However, the officer must have probable cause to believe that the item is evidence of a crime before they can seize it.