* Abstract nouns represent concepts, ideas, qualities, or states of being. An apology is a concept: it's the act of expressing regret or remorse. You can't physically touch or see an apology; it's an intangible thing.
* Concrete nouns refer to physical things you can see, touch, smell, taste, or hear. An apology is not a physical thing.
Let me know if you have any other words you'd like to check!