As we know, Mapbox is a suite of tools that provide a bunch of services. But those services can't be free forever or else they'ed be out of business! That's where an API Key comes in.
When using a basemap from Mapbox, an API Key is a way for Mapbox to know the request to their servers is authenticated and valid. Additionally, it gives them a way to track how much that key's being used.
Mapbox gives you a default key, which is okay to use, but we're going to create a new key that we can use specifically for our project.