If you play a lot, you probably didn't think of it once: "How did I know this." When you have such thoughts during the game, you know that someone has not led your design work to the end.
What's the point? The game should not allow the player to have enough information to make a decision that will allow him to win (or it is not possible to obtain this information in the game). For example, in strategy games, often one type of unit better fights other (stone-paper-scissors rule). The player should have access to this information in the game, e.g. when buying a infantry, tank or other vehicle.
Another problem is insufficient informing the player what the effects will have on his actions. eg in FPS games sometimes there are opponents resistant to the type of weapons. In this case, it is not possible to let our projectile-resistant monster krwawiło after each hit. The game should show projectiles that rykoszetują from it. In extreme cases, which I do not recommend, you can put a pop-up information about the resistance of monsters.
You can write hundreds of these types of examples.
Some games solve a low level of informing the player in an interesting way – by creating a Wiki. I'm not a fan of such solutions, as it forces people to open the browser and search for content. The best example of such a game let's be Minecraft, where many players have a problem with the construction of the simplest tools, or do not understand the operation of the world.
Remember: give the player enough information to be able to play effectively. If you can't use them, it's a problem.