Here's why:
* Concrete nouns refer to things that can be experienced with our senses (sight, touch, taste, smell, hearing).
* Abstract nouns refer to ideas, qualities, or concepts that cannot be directly experienced with our senses.
While you can't hold breath in your hand, you can *feel* it as it moves in and out of your lungs, and you can *hear* the sound of someone breathing. Therefore, breath is a tangible, physical thing, making it a concrete noun.