Hidden information refers to the faces of cards and other objects at which the rules of the game and format do not allow you to look.
Cards in the library, cards in the opponent’s hand, cards exiled face down (such as from Pyxis of Pandemonium), and cards other players are drafting, are all examples of hidden information.
Throughout the match, a draft, and pregame procedures, players are responsible for keeping their cards above the level of the playing surface and for making reasonable efforts to prevent hidden information from being revealed. However, players may choose to reveal their hands or any other hidden information available to them, unless specifically prohibited by the rules. Players must not actively attempt to gain information hidden from them, but are not required to inform opponents who are accidentally revealing hidden information.
Players can show their opponent any information that that player is allowed to see. This means they cannot show the opponent the contents of their deck unless they are currently allowed to see it (i.e. searching). Players cannot try to look at the opponent’s hand, but if the opponent has it revealed they don’t need to inform them. It is possible to accidentally reveal information. Maybe they are playing with their hand tilted too far forward, or maybe they are wearing reflective sunglasses.