I found it really hard to summarise CardStack because what they want to create is really revolutionary and the problems that they want to solve are really huge.
So, the video ended up 25 minutes long. In essence, CardStack is going to be a protocol that connects small code libraries, apps, services into huge products that have everything a user might need and yes you will be able to build those products yourself by picking components.
The token is needed to reward all these different software creators appropriately.