According to Sudoku rules, the number in a cell can appear only once in each block, each row, and each column. For each cell we can create a list of possible candidates that can appear in a cell. We can remove from this list of candidates any number that appears in the same block, row, or column. For some cells, the list of possible candidates may have only one single number. This is called a
"Naked Single", and this number should go into the cell.
The following is an example two
Naked Singles (see the picture):
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One Naked Single is number '2' (marked green) in the cell F5: numbers 1, 5, 7, 6, and 9 are in the same block, and numbers 3, 4, and 6 are in the same row F.
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Another Naked Single is number '7' (marked green) in the cell G8: numbers 2, 6 and 8 are in the same block, numbers 2 and 9 are in the same row G, and numbers 1, 3, 4, 5, and 9 are in the same column 8.