Signed URLs ↗ are a way to use Bannerbear to generate images on-demand using only URL parameters.
The standard Bannerbear product is a REST API ↗ accessed using an API key.
Signed URLs do not use the API key in the URL params as by nature these params will be publicly visible. Instead, the Signed URL uses an encrypted signature so that the API key is never publicly exposed, making them more secure, but requires a few more steps to use.
Check out the Documentation ↗ for more in-depth information on this.