While solving, you may want to note several candidate digits in a cell before committing to an answer.
Tap an already-selected cell to flip it into note mode; with a mouse, right-click any cell. The cell shows its candidates as small digits in a 3×3 grid, or a small checkerboard while it has none. Tap digit buttons to toggle individual candidates on and off.
Tap the cell again to flip it back to answer mode. If only one candidate remains, that digit is entered as your answer.