IOS License – Straight forward the license for the version of IOS you will be running on the ISR.
Feature License – Is the number of phones that the CUCME will be able to support. Think of it as attached to the ISR verses the phone.
Phone User License – Is the actually license for use and operating the phone. Think of it as attached to the phone verses the ISR.
Licenses can be added incrementally so you could buy a 24 phone license and later purchase a 48 phone license for a total of 72 usable phones.