What makes a Key Square important?

What determines what squares are "key" squares, and why is that important?

The usefulness of the square. If the square is useful for an attacking outpost or needed to protect the queening square, then it becomes a key square. As you become more developed, you'll learn that there are also corresponding key squares.

Key squares are squares where you can post a piece that is active and cannot be chased. Often it is the square in front of an ennemi pawn, the so called blockade square. This exerts a pressure on the opposing position, that can persist into the endgame. In particular in pawn endgames occupation of a key square often wins the game.

some of the "key squares" that are the easiest to find, are key squares for the knight, in the enemies position.

In pawn endgames they provide an immediate win because they enable the King to enter the scene decisively. Key squares provide a minimum of stability so the pawn structure is more or less fixed in that area.

So you should ask precisely:

-in which phase of the game
-which piece
-which pawn fomation
-concrete position

Otherwise it is as meaningless as „What is a good move?“

The way I first learned about this term was in a different context than what others are talking about. In pawn endgames, key squares are squares on which you should put your king to obtain opposition and queen your pawn. For example, consider the position below.

The key squares are c5, d5 and e5. Things are looking good for white, because his king on a key square, but he must be careful! If he plays 1. d4, the key squares will be c6, d6, and e6, and white will have no way to get there, because black will play 1. .. Kc7 and keep opposition. Therefore, white must play 1. Kd5 and grab a sixth-rank key square after 1. .. Kc7 2. Ke6 or 1. .. Ke7 2. Kc6.

I found this video by fantastic chess teacher Astaneh. I haven't watched it, but looking at the video length, it seems he explains the concept in detail.
youtu.be/OWSUcLKtqFc

This thread had an interesting sequence to get at key square e4
lichess.org/forum/game-analysis/how-to-get-the-black-king-to-e4-vlad-did-it

There are probably 1000 different examples of key squares and what makes them key.

In all variations, these are squares where a maximum amount of potential and opportunity is tied to those squares.
These are often squares where, if they're navigated correctly, you will win or prevent a loss.

Have you ever noticed an instance where a <insert piece here> moves <insert moves here> and then you have static difficulties because your opponent was allowed access to a square? That's a key square.

Have you ever noticed how a certain motif should have been used in order to gain a solid example, but because you found the idea a move late, your opponent has now blocked access to that move/square? That's a key square.

If there is an open e-file with your opponent's rook on it, and your king is on the g-file...it's very often the case that "Kf1", for example, would be the move for you to play in order to stop Re2. Rooks on the 7th/2nd ranks are often a very powerful motif; therefore, preventing the rook's infiltrating on the a2-h2 squares, or the a7-h7 squares, is usually a great idea when there is an open file. So those kinds of squares would be "key squares".

NOTE:

Q: "Which one is a key square then, Onyx, the Re2 or the Kf1?"

A: Both.

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Another example:

While there are openings that don't address the central squares immediately, playing these kinds of openings incorrectly, without careful concern for how to interrupt our opponent's pawn structure on those squares, is almost always going to be a failing mistake.

In these instances, playing as white, c4 and f4 might be key squares that our opponent would use to chastise the center at later stages in the game.

Here is a FLAWLESS example of a "key square" video, it contains a game with multiple instances of "key square" considerations.

One instance involves deliberately doubling pawns in order to prevent an immediate loss.

The other instance illustrates a theme involving tempo-defense/triangulation:

www.youtube.com/watch?v=ZcZ5tZ1qtI0

If you do not use a key square, your keys will hang lopsided.

@DrHack

I used to lose my key squares until I got a pocket chain.
My piano has never been out of tune ever since.