Full House/Last Digit

According to the rules of Sudoku, if a house (row, column, or box) has only one unfilled cell left, then there must also be only one digit missing from that house. Therefore, that cell can only be filled with that digit.

For example, look at row $5$ in the grid below: only $R5C2$ is still unfilled, and $7$ is the only digit missing from that row, so $R5C2$ must be $7$:

Full House can also appear in a column or a box. For example, in column $5$ of the grid below, only $R8C5$ is still unfilled, and $7$ is the only digit missing from that column, so $R8C5$ must be $7$:

Now look at box $6$ in the grid below: only $R6C8$ is still unfilled, and $2$ is the only digit missing from that box, so $R6C8$ must be $2$:

Full House is a special case of Direct Naked Single. You only need to inspect one house (a row, column, or box) to determine the only possible digit for a cell, whereas a Direct Naked Single usually requires considering all three related houses of the target cell: its row, column, and box.