The right of the people to be secure in their persons, houses,papers, and effects, against unreasonable searches and seizures,shall not be violated, and no warrants shall issue, but upon probable cause, supported by oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.
What it means?
The government can't simply go through your things without first going to a judge to ask "please, we think this person is a criminal and here's why we want to look through his things, his mail, and/or tap his phone."