OK, this one is a bit arid and difficult to understand if you don't know (or take the time to find out) what the terminology used means but, basically, what the study says is that birds have skulls that evolved differently from the reptiles once they separated from the dinos because they needed to be very apomorphic (meaning, highly specialized to fulfill specific needs), that they are paedomorphic (meaning they retained baby features) and that it happened in four different stages.
https://www.nature.com/articles/s42003-020-0914-4