I think the most basic difference between the two is how Tapas Originals are made by the creators in collaboration/partnership with Studio Tapas, whereas premium works are completely made and run by the creators.
When you have a premium series, your episodes get locked behind a paywall and the readers pay ink to unlock them of which you get a certain percentage and that's as far as your relationship with Tapas goes.
With original series, Studio Tapas not only helps you create your work from scratch, they also own the IP towards it. In return, Studio Tapas makes large investments towards your story by providing you various resources and connecting you with editors and also by marketing it.
This help article explains what premium is https://help.tapas.io/hc/en-us/articles/360007617294-What-is-the-Premium-Program-
And to get a better idea of Tapas Originals you can check out the Studio Tapas site https://studiotapas.com/
