When we look at a tic-tac-toe board, we look at each row, column, and diagonal. Essentially the question is "are there three squares in a line that contain two pieces that are mine with one square empty?" In fact nothing really matters about the game besides the potential winning combinations.

There are eight winning combinations in tic-tac-toe. By looking at our previous board:

We can then make a list of all its winning combinations, or triples for short.

Take a minute to convince yourself that this list is all you need to know about the corresponding board position. Once the strategy finds the triples for a board position, it never needs to look back at the original position again.