The Know-It-All Brothers are three Kokiri brothers in Ocarina of Time who serve as guides on how to use some of the basic functions in the game. Their house is located at the west end of Kokiri Forest, near the Forest Training Center. One of the boys will teach you about the various icons at the top of the screen. Another boy will teach you about the map and the three different types of items; equipment items, C-Button items, and Quest items. The third brother will teach you how to use the C-up button to talk to Navi, and to change the camera view.
The three brothers never leave the house throughout most of Link's adventure. Even when Link returns to Kokiri Forest as an adult, the three brothers are still in their small house. None of the brothers recognize Link when he comes back as an adult, and they state that they are awaiting the return of Link.
It isn't until the end credits that the brothers finally leave the house. They can be seen at Lon Lon Ranch during the End Credits celebration. Strangely enough, the Kokiri children all seem to be perfectly fine in Lon Lon Ranch, even though it is stated that the Kokiri children cannot leave the Kokiri Forest or else they will die.
While there are three brothers who live inside the house, there is another Kokiri who is right nearby, just outside the Forest Training Center. He teaches Link how to perform some simple jumps while targeting an object. Given that he is right nearby the house and teaches Link basic game mechanics, it's possible that he is another one of the Know-It-All Brothers.