What are LUFS and why is Spotify/Apple 'limiting' my podcast?

Do you make content with audio? 

You may think that you can make your music or sounds as LOUD as you want to, but Spotify, Apple, and most streaming services will change how it sounds. Learn the basics of LUFS and how they affect everything you create!  

Presentation starts at 3:20.  

"What's the Deal with LUFS" - Created by Finnur Nielsen for Fool & Scholar Productions

What are LUFS and why is Spotify/Apple 'limiting' my podcast? (YouTube)