Planar Atlas is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

This artifact enters tapped. When this artifact enters, you may look at the top four cards of your library. If you do, reveal up to one land card from among them, then put that card on top of your library and the rest on the bottom in a random order. : Add .