Stephen Covey wrote a book with this title, urging effective managers and individuals to work on basic priorities in life.
How does this apply to chess, especially for someone who's young and just started?
I would say KNOW YOUR SQUARES AND DIAGONALS.
Chess involves a player being able to make a move and anticipate his opponent's possible reply, then making another to counter that reply. The process is known as calculation. This ability can be improved and strengthened if the player has good grasp of the squares on the board and the lines and diagonals without having to see them on the board when calculating.
Many children playing tournaments I notice dislike recording their moves. Why is this? They have trouble finding the squares of their moves, whether they are playing White or Black. Few take the trouble to sit down and learn them to know it well. However, I realise that those who do have much better board vision than those who don't. Interesting.
Knowing the name of each square and where they are located links chess study to how the brain "sees" the board. Stronger players do not actually visualise the entire chessboard in their mind and have graphic images of pieces when they calculate moves ahead. It's more like a blind man who used to see knowing his way around his house - he can sense where the kitchen or bathroom is, noting how many steps there are to get from the living room. He'd know where the sofa is placed, or the lampshade next to it.
So we relate this to piano playing, its like knowing your scales. Or in mathematics, your multiplication tables. You know it so well that you don't have to think about it.
Once this ability is mastered, I assure you that you will see huge benefits - less mistakes, more alertness to possibilities that can occur during the game. The best benefit of it all, children can now enjoy reading chess books and improve at their own pace!
My generation never had the benefit of coaches so most of the players among my peers are self taught using the descriptive notation. It's even more complicated than the current algebraic notation, but we got by to use it to replay the great games of the masters. When algebraic notation is introduced it was a breeze for us to pick up as there's only 1 dimension to work on.
So if there's a priority of chess skills to enhance vision on the board, get the kids to be able to locate the square on the board (without looking at it of course). Start with them doing it when White, then with Black. Or they can play battleships by drawing a 8x8 board and attempt to guess each other's ships by calling out the coordinates of the squares. These fun activities can help the young player learn the squares, then move on to learning every diagonal, every possible square that a Knight can reach from where it is and so on.
Board vision can be mastered, in fact it is one of the most important skills to work on in my opinion. Perhaps you will like to see what GM Kevin Spragett has to say on this by reading his thoughts in