Notes
A note is a small candidate written in a cell you have not solved yet. It means “this digit might still belong here.” It is not the answer. The large digit you place with Notes turned off is the answer.
Turn notes on with the Notes button. It reads ON or OFF. While it is ON, tapping a digit in the selected cell adds or removes that candidate. While it is OFF, tapping a digit fills the cell. Erase clears the cell’s answer, or its notes if that is what the cell is showing.
Which candidates are worth writing
Look at the row, the column, and the box that cross the empty cell. Any digit already sitting in one of those three places cannot go in the cell. The digits that are missing from all three are the candidates.
Example: a cell’s row already shows 1 and 8, its column shows 2, 5, and 9, and its box shows 7. Those six digits are eliminated. The notes worth keeping are 3, 4, and 6. Writing 1 as well does not help, because the row already has a 1.
Auto-Clear Notes
Settings includes Auto-Clear Notes. When it is on and you place a 4, the game removes 4 from the notes in that same row, column, and box. The other candidates in those cells stay. When it is off, you delete stale notes yourself. A note left behind after the digit has been placed elsewhere is a common source of a wrong fill later.
You do not need notes in every cell at the start. They are most useful in rows and boxes that are almost finished, where two or three candidates remain. Once a cell has a single candidate left, that candidate is the answer. That case is a naked single.