What's an API Key?

[Antwerp]

Never heard of this but it seems to be for the Google Maps feature.

[Bananarama]

Only required if you plan on more than 10k worth of free image downloads per day when building a route (doubtful anyone would even come close to this number), you can continue using the Google Maps overlay without an API key. However, to extend the free downloads to 25k, you'll need the Google API key and enter it in appropriate field in Railworks.