Postman which I thought worked locally on my computer, is down

I'm stealing your thread to ask a question: How would you explain what an API Client is to someone who's never used one before, and has always just consumed/produced/tested API's the old-fashioned way?

Is it basically "an IDE for playing with API's"?

Is it only for HTTP-based API's?

Does it come with canned functionality for popular services out there?

I read this but still don't feel like I fully understand what it does https://www.postman.com/api-platform/api-client/

EDIT: This blog post https://schier.co/blog/call-for-beta-testers made it click a little better: "...app that makes testing REST APIs super easy"