Things that are subjective are open to interpretation. If you go see a movie about a jewel thief, the topic is not subjective. But whether it's a good movie or not is subjective.
Subjective things depend on your own ideas and opinions: there isn't any universal truth. Subjective is the opposite of objective, which refers to things that are more clear-cut. That Earth has one moon is objective — it's a fact. Whether the moon is pretty or not is subjective — not everyone will agree. Facts are objective, but opinions are subjective. What's the best song, band, movie, or TV show? These are all very subjective issues. There's no right answer.