If you're a typical front-end developer, you'll work with npm a lot. One of the tools npm provides is called npx. I've seen it used a lot in tutorials and got tired of not knowing the difference!
While npm doesn't actually stand for anything, it's commonly accepted to mean "node package manager".
It turns out npx is the "npm package runner". I guess the "x" comes from "(e)xecutor" or something?
The official blog explains:
npx is a tool intended to help round out the experience of using packages from the npm registry — the same way npm makes it super easy to install and manage dependencies hosted on the registry, npx makes it easy to use CLI tools and other executables hosted on the registry. It greatly simplifies a number of things that, until now, required a bit of ceremony to do with plain npm.
For more information, check out the post on the npm blog: