Here are some ways it's used:
* "The police have ruled out robbery as a motive in the murder." This means they no longer believe robbery was the reason for the murder.
* "Doctors ruled out a heart attack, but they're still investigating other possibilities." This means the doctors don't think a heart attack was the cause of the patient's symptoms.
* "I ruled out going to the beach because it was raining." This means the speaker decided not to go to the beach because of the rain.
In essence, to rule something out is to remove it from consideration. It's a decisive act that usually implies a reason or justification for excluding the possibility.