"Last Digit" Sudoku technique explanation:

Singles are the most basic and easy-to-understand techniques in solving sudoku puzzles. The Last Digit is the easiest and most understandable of them all.

It simply means the last digit that can be placed to correctly solve the Sudoku puzzle.

An example:

We can see that there is only one empty cell that remains in this number puzzle, it is marked with a blue square.

Take a look at the house marked with the blue square, the only missing number in this house is 1, so 1 is the last digit.

A house is a full set of numbers between 1-9 it can be a row, block, or column. If it is not the last digit, it is often called "Full House"