How it works
A digit can only fit in one cell within some row, column or box, even though that cell may still show other candidates too. Since the digit has to go somewhere in that unit, and this is the only cell that can take it, it belongs there.
Try it
Every digit 1–9 must appear exactly once in column 6. Let's pencil in what's still possible for its empty cells, then find the one place digit 1 has to go.
Tap an empty cell in column 6, then tap every digit that's still possible there — check what's already placed in that cell's row, column and box. Press "Check my work" when you're ready.
Nice — every candidate in column 6 is confirmed.
Not quite — look again at what's already placed in each cell's row, column and box, then check again.
Now let's scan column 6 for digit 1.
Row 2, Column 6: 1 isn't a candidate here — ruled out.
Row 3, Column 6: 1 isn't a candidate here — ruled out.
Row 4, Column 6: 1 isn't a candidate here — ruled out.
Row 5, Column 6: 1 isn't a candidate here — ruled out.
Row 7, Column 6: 1 is still possible — and it's the only cell left in column 6 that can take it!
Since 1 can only go in Row 7, Column 6, tap that cell and place it.
That's it — nicely done.