Gwent is a collectible card game available in The Witcher 3. The game features at times during the main story, but for the most part functions as an optional side quest. The game sets up the players as opposing generals of two armies, using customized decks to try and outmanoeuvre each other. The cards for the game are obtained from merchants and from winning games, with certain opponents dropping rare cards. There are also Gwent tournaments that are held in Novigrad.
Games of Gwent can be quite short and so the tutorial game that is provided can leave the player feeling a little lost. I had to play several more games before feeling comfortable with the mechanics. However the games designers, Rafał Jaki and Damien Monnier, have now provided a tutorial video on youtube which explains the game mechanics, the special effects that certain cards have, as well as detailing certain tactics that can be used. The tutorial video can be found here https://youtu.be/Hhnhf8VMmhM.
If you have not played Gwent in the Witcher 3 yet, it is well worth investing your time in it. I have found myself spending hours seeking out merchants and other players to play against, and with this new tutorial there is really no excuse not to try the game for yourself.