As Eli said, Electron is covered by an MIT license. It describes itself with this sentence:
Build cross-platform desktop apps with _javascript_, HTML, and CSS.
It accomplishes this by uses Node.js, a _javascript_ runtime built on the V8 _javascript_ engine of Chrome. Each Electron app comes with its own integrated copy of Chromium (the "free" parts of Google's Chrome web browser, released under a combination of 3-clause BSD, MIT, [L]GPL, and MS's Shared Source licenses). Without intensional malice, I'd call it a Frankenstein's monster of both code and license.
Its major features are (in my personal opinion): Google invests effort into making it function well across major platforms, it's "free enough" that it can be used by various open, free, and commercial projects, and it lets people make "desktop apps" using the widely-known web stack of _javascript_, HTML, and CSS. It can use native code via Node's analog of an FFI. This combination is so widely spread and optimized at this point that it's possible to get decent performance alongside gui, threaded, network, etc. code. It also has a reputation for being fairly bloated, in large part because each Electron app brings its own copy of chromium, V8, node, etc. Similarly, Electron apps are rarely well-integrated into a particular OS, since they're mostly WWW technology; whether this bothers users or not is currently a shifting topic, as ever more new users are used to using web-based tech for much of their computer needs.
I hope that helps,
~Chad