what are class ex-cube? How to use them?

I tried to google it, but I am not quite understand the uses of it. Can anyone explain please?

There are some quests from your trainers that give you more SP for class ex-cubes. Think you need 10 in total. Other than that you can exchange for standard ex-cubes - but I'm holding onto mine as I read somewhere that, when we get further updates, we might regret trading them in