I'll let Wikipedia explain Reddit.
I just know Reddit is a huge, famous social site that has gaming celebrity interviews sometimes. You don't have to do much with Reddit for the giveaway. The sign up for this freebie is very simple.
Just make an account, which takes about 10 seconds, and while signed in follow these instructions from the same page- Dark Eye Giveaway from Reddit, GoG, & Dev
how to get your free copy of The Dark Eye: Chains of Satinav:
Send a PM to user /u/GOGaway. Keep it simple, since we're not going to be reading the PMs here. Maybe you wanna tell GOGaway what you did last summer, or your secret plans for world domination. Whatever you say there, we won't judge.
Wait. We have 40,000 codes to give away and Reddit's API recommends a 3 second delay between each PM; that means that it will be, like, a minimum of 36 hours until we give all of the keys out. Don't worry, though. We got your PM. I totally checked.
The bot will read the mail and send you a PM with a GOG.com key for the The Dark Eye: Chains of Satinav.
Yes, you are absolutely right. The new game they are celebrating is an RPG but the game they are Giving. Away. For. Free. is a point and click adventure that normally sells for $19.99
You can see the game they are giving away here - GoG: Dark Eye: Chains of Satinav
I don't know precisely what the letters AMA stand for, but it is describing a question and answer session they recently had with the developer who made both the new RPG they just released and Dark Eye: Chains of Satinav
The only times I have looked at Reddit in the past was to read one of these sessions (after it was over. Which is why I never joined before.)
I hope that helps?