Each time this action is called it gets the next item from a Array. This lets you quickly loop through all the items of an array to perform actions on them.
In this example we want to run the same set of actions on 4 wheels (GameObjects). First we make a Game Object Array that contains these wheels. We can then use Array Get Next to loop through them one by one. Each time round, the targeted wheel is stored in a targetWheel variable that can be accessed by other states/actions.
NOTE: You can run through many states in a single update. So you can also think of FSMs like a flow chart with each state describing a step in the process. See Core Concepts.