The MacArthur Causeway is one of a number of large roads that connects Miami Beach with mainland Miami. It's the southern-most of these roads and is the one that's closest to downtown Miami.
The causeway has water on both sides of it. On the south side, you can see a lot of shipping cranes and oh my god about a million cruiseships all lined up in a row. Six massive cruiseships, all lined up one after another, give an impressive impression.
On the north side there are bridges that periodically branch off from the causeway to a number of man-made islands: Star Island, Palm Island, Hibiscus Island. The islands look like they're mostly private, with large houses that have their own piers with a yacht parked in front of each one. Some of the yards of these houses look like overgrown jungles with crazy mansions peeking out from among the palm trees.
There are a lot of busses that go fairly frequently across the Causeway, and they even go pretty late at night, making bus a good way to go between Miami Beach and Miami.