API
aka: endpointaka: rest
The Quick Version
A structured way for two pieces of software to talk to each other.
How it actually works
An API is a defined set of requests one program can make to another, plus the shape of the answers it gets back. Your frontend calls your backend's API over HTTP; your backend may call third-party APIs like a payment provider's.
Why does it matter?
Whenever your site loads data or saves something, an API call is what makes it happen.
Example
POST /api/estimates with a name, email and message creates a new estimate request.