The issue is a flashpoint in the PC world. It's pretty much the exact same thing as the online retailers themselves, just for user created content. As a gamer, you pick up credits through Steam, PlayStation Plus, or Xbox Live, and then you spend those credits on the mods you want to buy on the Creation Club's storefront page.
It's a simple name for an idea that stumbles into a very complex issue in the gaming world, one in which the creators of mods have the option of making money off of their creations. One of the biggest moments of Bethesda's E3 2017 press conference wasn't the introduction of a video game, but rather, the introduction of a new, officially backed mod community, Creation Club.