How it works
A cell has only one candidate digit left — every other digit 1–9 is already used somewhere in that cell's row, column or 3×3 box. Whatever's left is the only digit that can possibly go there.
Try it
Let's find out why Row 5, Column 6 can only be one digit. Tap every digit that's still possible there — check what's already used in its row, column and box.
Tap every digit that's still possible in Row 5, Column 6 — check what's already used in its row, column and box. Press "Check my work" when you're ready.
Nice — every candidate for Row 5, Column 6 is confirmed.
Not quite — look again at what's already placed in each cell's row, column and box, then check again.
Only one digit survived: 1. Since every other digit 1–9 is already used somewhere in Row 5, Column 6's row, column or box, 1 is the only one left.
Since 1 is the only candidate left, tap Row 5, Column 6 and place it.
That's it — nicely done.