A sequel is a continuation or part two. The Empire Strikes Back and Toy Story 2 are sequels. Many successful movies have sequels.
The most common meaning of sequel is for a book or movie that follows another, but the term can be used for just about anything that comes second. We might say of an actress who begins her career with a bang, "What will she do for a sequel?" A football team that wins a championship may have trouble with the sequel — trying to win another one next year. All sequels have a tough challenge — living up to the original.