5 endpoints, prefilled, run against your own account. Requests here are real: they spend the same credits and return the same bodies your code will get, because they go through the same functions. A new account starts with 500 free credits.
There is nowhere to paste a key, on purpose. The playground runs on your session and prints the curl you would run with your key, so a live credential never has to go into a text box in a web page.
Once a call does what you want here, you need a key to make it from your own code. Open the API page, name a key after the thing that will use it, and press create. It is shown once, so copy it there and then. Authentication covers rotating one and what to do if one leaks.