GOG.com is proactively adding more security to every gamer's account.
If you have an account on GOG.com, you'll send have two-factor authentication protecting it, and you don't have to do a thing. In a press release and mass email over the weekend, CD Projekt announced that it will roll out the new security feature for every user.
If you've used two-factor authentication before, the process will be very familiar to you. The first time you log in to your GOG.com account, you'll get an email with a code in it, and you'll enter that code to continue. The official announcement says "If you're using GOG mostly from home or work, you should only be required to use the Two-step code very rarely."
The company says that you are not required to use the two-factor option, but it will be on by default. You do have the ability to opt out of the feature by clicking this link, or by disabling it in the Account Settings after it goes live on October 24.
If you want to get all the info on the new feature, you can find it in this FAQ on GOG.com.