As said before, the map is a pairing between a key and value. The Map constructor expects an iterable collection of key-value pairs. In the preceding recipe, we've passed a two-dimensional array. The outer dimension is the iterable that contains multiple entries.
The inner dimension is the key-value pair. The first member of the key-value is the launch site. The second member is the value (in our case, a Rocket). The Map constructor iterates over the entries provided and creates a pairing between each.
