Single Candidate

If we fill in all candidates, we see immediatly that some squares have only one candidate.

In the example, it is 2, 4 and 7.

As soon as you fill in them, you can eliminate them in other cells and see that there are again some single candidates left.

Look at the highlighted line.

What can you place in the empty squares?

In the first, 1 and 2 are still possible.

In the other, only a 2. So, we will place this.

We know now also that the only candidate left for the previous cell is 1.