Howdy!
So to keep it simple ink is tapas' site currency. As a reader you can use it to support other creators or unlock episodes for premium series. As a creator when someone supports you with ink it's converted into actual money that you can withdraw.
This is a link to Tapas Staff's comic on how to get ink:
and if you're looking for more details on what ink you might be getting as a creator you can check the Support and Revenue tabs of your Creator Dashboard, which you can get to from the drop down menu from your profile icon on Tapas main site.
Hope that helps
