Define Beta Boy Please?

So Elle's 30 day character challenge has Favorite Beta Boy coming up. Being very new to all this, I've never seen this term. I think I know what it means, but Google isn't helping me feel sure about it. So, my lovely MDLers, What exactly is a Beta Boy? 

On a similar note, I misread Chaebol as Chaeboi. Honestly I think it still works as a KDrama 'poor little rich boy'. Is this just my head or has someone already coined it?

Thanks!