They give you coins. The first tier of coins is 50 coins a level. Two levels gives you 100 coins with which you can buy a free pack from any card deck you want. OBS is a free streaming program that many people use. It will also just record without streaming. =) I don’t know what Blizzard’s policy is about links so I will just say do a search for OBS on the internet and it will take you to their web page (not a Blizzard program).
Have a great time!