Yes it's a little confusing and it could use a formal definition but "Premium apps" there just means the apps that are both
1) made and supported by Splunk,
2) commercial products, ie not free apps.
Good examples are the "Enterprise Security" app and "Splunk for VMWare 3".
So in the world of "non-premium" apps, there are then various free splunk-owned apps like Splunk-on-Splunk and the Unix app - since those are free they are not Premium apps so you can use these with the Splunk Free License.
And Likewise there are commercial Splunk apps made by third parties like Sideview and although these are not free apps, you can use them with the Splunk Free License and some customers do.