medium tier
Look at the pencil marks in the two highlighted cells, both in the same 3×3 box. Digit 3 only appears as a candidate in these two cells within the box — and they happen to share a row.
Since 3 has to go in one of these two cells, the rest of that row can't be 3 — so 3 can be crossed off the marked cell, even though it still has other candidates too.