When you create a blueprint it asks for what other blueprint to use as the parent. That is often an Actor, or another “common class”. But you can pick any blueprint as a parent.
So a blueprint “is a” whatever its parent class is, and than adds to that.
So if the parent blueprint is a Vehicle, then the blueprint may be a Car.
The Vehicle has MaxSpeed, NumberOfWheels, GasTankSize.
The Car then might add the TurnRadius is 10 meters. etc.
So a Car “IS A” Vehicle.
In Unreal Engine, maybe your Soldier blueprint’s parent is an Character.
So the Soldier IS A Character, and the Character IS A Actor and Actor IS A Object.
Object is the base class or parnet of ALL blueprints.
In effect the parent blueprint is nested inside the child blueprint.
Variables in a blueprint are a HAS A relationship. A Vehicle HAS A NumberOfSeats.
A Soldier HAS A HitPoints.
IS A is called inheritance.
HAS A is called Agregation.
In both blueprints and C++ classes there is inheritance.
A blueprint is a template for an actual instance of the blueprint.
So Soldier might be the blueprint or template or cookie cutter, and ChuckNorris is an actual specific instance of a Soldier (the cookie). When you add a blueprint to a map or scene in your game you are using the blueprint to create an actual instance.
Ok?