ACR clash of clans freeroll

does anybody know what these are all about I see on the web site you win gems but what are they for and how do you use them..

We're running Clash of Clans Freerolls with a total of 10 tournaments running at 7:00pm and 2:00am EST from July 23rd to August 10th. Top 5 players in each freeroll will receive a Sack of Gems (2,500 Gems).

Clash of clans is an epic combat strategy game and Gems are the official currency used to build villages, upgrade buildings, or increase a hero’s healing rate and more.

All you need is an Americas Cardroom Account and you'll have lots of chances to conquer the gems.