Creation Club is a new idea from Bethesda that will start with The Elder Scrolls: Skyrim and Fallout 4. They're mods that are created by third party developers and individual modders that will get some help from Bethesda themselves. By doing this it means the mods can be compatible with all saves for the game rather than having a chance to break your game. These mods will also be sold for points, which can be purchased by you.
On the website for the Creation Club there is an FAQ explaining more about it. Mods will only be available for Fallout 4 and Skyrim Special Edition. All mods on it will be curated and will feature mostly the same things you normally see from mods we already get. You have to apply to have your work featured on the Creation Club, and you need to have work ready to show them. Once you're approved you'll have an alpha, beta, and final release milestone set for you. Bethesda will pay the modders for their work when they hit those milestones.
It should be noted that Bethesda will not allow any mods to be retrofitted for the Creation Club and all mods must be new and original. All of the content will be approved internally by Bethesda and has to go through a similar development and approval process to any content the actual company would have to make. Bethesda is also insistent that this is not paid mods, but you do have to buy credits to pay for the mods. However, it could be said this is more like opening up new DLC for the games for any third party, since all content has to be curated by Bethesda.
Creation Club will be available on PlayStation 4, Xbox One, and PC some time this summer.
If you want to know more about this and other announcements happening at E3 then be sure to check out our E3 2017 Coverage Hub.