Certain game affecting character attributes are hidden from the player, making players either completely oblivious or only vaguely aware of their existence. Also stats that are stated to exist, but nothing clear is given for what their values are.
The first video game about Hidden attributes was released in 1985.
Reasons for hiding this information can be any number of things, first and foremost comes immersion because it's something you wouldn't know for yourself either, another is to make metagaming harder and make the player more reckless (or careful if they understand it) because they have no direct way of telling how much more they can do something before they're punished for it (e.g. karma), possibly (though unlikely) an oversight from developers, etc.