When an empty cell has only one candidate left, that is its digit.
Where to lookA pointer first: look at row 3, column 9. Which digits can still go there?Naked singleOnly 6 is left for row 3, column 9.
First tap "Candidates" to show what each blank cell can still take. Look for a cell left with a single digit. Digits already in its row, column or box cannot go there, so enter the one that remains.
This example, step by step
Where to lookA pointer first: look at row 3, column 9. Which digits can still go there?
Naked singleOnly 6 is left for row 3, column 9.
In hints
The hint points to the cell first, asking which digits it can still take, then shows why only one is left.