This might be the most common and basic technique to start with while solving a Soduku puzzle. When trying to determine where a digit should be placed in a row, a column or a 3x3 box, we may find that there is only one cell this digit can go and therefore it has to be in that cell.
Consider the following partial Sudoku puzzle:
The yellow cell must be a 2. Why? Let us look at the 3x3 box in colors. It is because this is the only cell in this box that can hold the digit 2. So the digit 2 must be in this cell. Those six blue cells in this 3x3 box cannot have any 2 since there has been another digit 2 in the same row respectively. Also those two green cells cannot have 2 since they have been occupied by other digits. Therefore the digit 2 has nowhere to go except the yellow cell.