Imagine you are getting dressed. You must put on socks before shoes. You must put on underwear before pants. You must put on a shirt before a jacket. You cannot put on shoes first. A topological sort is a linear ordering of tasks that respects all of these dependency rules.
If means " must come before ", then appears before in the sorted list. Given a directed graph of dependencies, your goal is to find an order that satisfies every edge. This section will show you how.
What does a cycle mean here?