The MessageMedia Webhooks allows you to subscribe to one or several events and when one of those events is triggered, an HTTP request is sent to the URL of your choice along with the message or payload. In simpler terms, it allows applications to "speak" to one another and get notified automatically... More information
The MessageMedia Signature Key API provides a number of endpoints for managing key used to sign each unique request to ensure security and the requests can't (easily) be spoofed. This is similar to using HMAC in your outbound messaging (rather than HTTP Basic).