All easy and medium difficulty puzzles can be solved simply by using the single candidate
and single position
Sudoku solving techniques. To solve the harder Sudoku puzzles you will need to use pencil marks.
Look at each cell in the grid in turn, and write in small numbers all the candidates for that particular cell. If you were doing this on paper, you would use a pencil so that you could erase it later, hence the term 'pencil marks'.
Pencil marks themselves don't help you that much, but they do help you to use the more advanced Sudoku solving techniques