You learned sets. You understand elements, subsets, union, intersection, difference, and complement. You can read set notation and apply set operations in code.
You walked through four problems showing how sets solve intersection, difference, mismatch, and consecutive sequence tasks. Each problem used sets to speed up brute-force approaches.
Good job on finishing this section. Now go practice the problemset. The next section covers functions, which build on everything you have learned so far.