What is meant by "Outer Object"?

Construct Object From Class

What does OUTER object mean for this blueprint node?

It’s the owner of the object. Every object in UE needs to have a reference pointing at it, “someone have to know it”, because objects are garbage collected and if nobody knows the object, it can’t be collected and removed from the memory if the object has been destroyed. This does also mean if the outer respectively owner object gets destroyed the “Image Plate Component” gets destroyed also.