The //MARK: syntax is used to navigate easily between different parts of your code. Let's see what it does:

1. Look at the path that is visible just under the Tool Bar and click on the last part as shown:

2. A menu is displayed, and you will see both Private Extension and UICollectionViewDataSource in it, generated by the //MARK: syntax. This enables you to easily jump to these sections:

You are done cleaning up ExploreViewController, so let's do RestaurantListViewController next by refactoring it and adding extensions.