CORS
also Cross-Origin Resource Sharing
A browser security mechanism that controls whether a web page from one origin is allowed to make requests to a server on a different origin, governed by server response headers.
Analogy
Like a bouncer who only lets in requests from guest origins on the approved list.
Why it matters
It's the usual culprit when browser-based calls to an API or webhook get blocked, so understanding it unblocks frontend integrations.
In practice
You add an Access-Control-Allow-Origin header on your API so a funnel page on another domain can fetch from it.