My first UE4 tutorial with awesome cat(s) included :p
Tutorial tried to teach you about:
-That is class and how C++ and Blueprints are connected with them
-What is class hierarchy an inheritance
-What is API reference and how to swim in it
-Difference about UClass and object instance (but i think i little failed on that, but it's not so common problem)
-How to make actor reference and setup object variable
-How to make function calls from actor references (without need of interfaces)
-What interface (truly) is
-About event overriding and how to deal with (but side effect of override didn't work for me, so just in case)