* Concrete nouns refer to things that are tangible and can be perceived by the senses (sight, touch, smell, taste, hearing). A footprint is a physical impression left in the ground, so it can be seen and touched.
* Abstract nouns refer to ideas, concepts, qualities, or states of being. They are not physical objects and cannot be perceived by the senses.
Therefore, a footprint is a concrete noun because it represents a physical object.