Webhook is a link that when entered will work as a trigger to fire your macro.
Note that there are multiple apps that can 'enter the link' using http request action (its macrodroid also).
I am for exemple using webhooks to communicate between my devices: my 2 phones and pc.
I can remotely trigger macros on my second phone (or main one) or unlock my main phone using my pc etc.
macrodroidforum.com
Oh and about discord, I am not sure what what they do exactly but you probably could use send http request action to 'control' your discord using macrodroid - sending messages etc.