Safari Technology Preview is different from the Safari browser you find on your Mac (or in the iPhone and iPad) in that it’s aimed at Web developers. It offers a bleeding edge look at incoming technologies; think of it like Chrome’s various channels (stable, beta and dev) only Apple chose to break its ‘dev’ channel out into a standalone app.
We knew Apple Pay was on the way to Safari in macOS, but developers can now start poking around at it for use within their own pages. There’s nothing user-facing that can be accessed yet, but there’s enough there for Web developers to get excited about.
macOS Sierra is still in developer preview. It’s not clear when Apple may open up the public beta for Sierra, but don’t expect to be able to buy things with Apple Pay on the Web when it does pop up. Apple Pay will likely be kept under wraps for public consumption until Sierra gets its proper, stable release later this year.