Read and write custom application data in Dynamics CRM Online Basic

Hi,

I've got a lot of doubts with Dynamics CRM Online versions.

I know that with Professional version, you can do anything, but I don't know what you can do with Basic and Essentials.

In Basic and Essentials Microsoft says "Read and write custom application data" What this mean?

I can create new entities?

In Basic and Essentials I think that you don't have quotes, Bulk Import, marketing campaing, products.. but Can I create them using new entities and relate with Accounts, Contacts and Opportunities?

Can I do a license mix? For example 1 Professional and 3 Basic?

Thanks