Knowing is half the battle. Knowledge of where your objective is simplifies getting to it by eliminating any possible false paths. But hidden information is different than just a straight unknown. Hidden information is when one side has knowledge that the others do not.
If no one knows where the objective is, we’re all on equal footing. If it’s a pre-game setup, then it’s randomness. If it’s up to the skill of the player, it may be player, solver, or perception uncertainty (we’ll get to perception uncertainty soon). But when one side has the information, and another does not, it becomes hidden information.
This type of uncertainty adds an element of sly play and espionage to a game. Poker is the ultimate form of this, where skilled players read each other and calculate their odds in order to win. Many board games have similar mechanics, with hidden objective cards or even deduction puzzles.
RPGs rarely use hidden information, but when they do, it can be the surprise twist, or player vs player conflict that is the highlight of a play session. I would count the GM as a neutral entity, much like a random game setup or a 3rd party puzzle master. Their “known” information is hidden from the players, yes, but it’s also subject to change, making it much more of a narrative uncertainty (which we’ll get to later as well).
A common form of hidden information in RPGs is a betrayer, or person goals mechanic. Generally, smaller, more focused games have mechanics like this, so that the scenario allows that hidden information to be meaningful. But, there’s no reason a long campaign can’t end in an unexpected twist that the GM and another player have been in on since the beginning. But be careful with hidden information in RPGs; it can make or break a player’s experience with the game if it was an “unpleasant surprise”. Make sure the outcome generates more fun than anti-fun.
Not everyone digs hidden information. It can certainly muddy the waters and make the game feel lopsided, even if in theory all players have equal portions of hidden information. It can also feel random, as even the best poker players can be bewildered at the flip of the cards. But it can also keep tensions high until the very last minute, making the climax of any story a memorable one.