Reify is a general term, it means to "make concrete" (or to "make real" depending on the usage) something that is previously fuzzy or abstract.
When you make a concrete subclass of an abstract class, you are "reifying" that class. When you made the abstract class from the concept of something, you are "reifying" that concept.
When you make a concrete subclass of an abstract class, you are "reifying" that class. When you made the abstract class from the concept of something, you are "reifying" that concept.
It's a fun word.