It refers to a word that names a person, place, thing, or idea.
Here are some examples of sustantivos in Spanish:
* Persona: hombre (man), mujer (woman), niño (child)
* Lugar: ciudad (city), país (country), casa (house)
* Cosa: libro (book), mesa (table), computadora (computer)
* Idea: amor (love), felicidad (happiness), libertad (freedom)