Keeping Track Of Candidates
When you start to solve a Sudoku, all you see are the numbers that were given to you, like in the image below.
The rules of Sudoku mean that you can't place your numbers anywhere. If a column already has a certain number in it, that number can't be used again in the same column.
Using that information, we can take our grid and write in all of the numbers that could be used in each cell. We call these candidates.
In this image, the small numbers are the candidates. We can now see that there are 3 cells (marked with a red dot) that only have one candidate. We now know the value to write in that cell.
Once we enter a value, that will mean that we can't use that value again in the same row, column or minisquare. We should eliminate that candidate from all other cells in the same row, column or minisquare.
Keeping track of the candidates for each cell is essential for completing the harder puzzles.