API is application programming interface, a structured way for software systems to communicate and exchange data.
It matters because development decisions affect speed, security, scalability, tracking, SEO, and maintenance. A site that looks good but is fragile or hard to update can quietly hold back every marketing channel.
Example
A booking site requests available time slots from a scheduling service through an API. It must handle authentication, response formats, unavailable slots, and errors instead of treating every request as a successful booking.