Every row, column and 3×3 box holds the digits 1 to 9 exactly once. So once eight of a unit's nine cells are filled, the last one cannot be anything except the digit that is missing. There is nothing to work out — you read the eight, note which digit is absent, and write it.
This is the first technique worth naming because it is the one that cascades. Filling that last cell often completes a crossing row or column, which completes another, and a single entry can unlock five or six more in sequence.
On this site those cells are filled for you the moment they become certain, in both the unit you completed and any unit the fill went on to finish. It is the one place the board does work on your behalf, because clicking a cell whose answer is already forced is bookkeeping, not solving.