“Pantalones" is plural in Spanish, and "uno" is singular. So we are essentially saying "a pair of pants" in Spanish by pluralizing uno in "unos pantalones." If I’m not mistaken this can also be used as “some” or “a few”. For example, unos días would be a few days. Also when used with a masculine noun, uno becomes un. For instance, un hombre not uno hombre. I’m posting this here just to clarify my understanding and in case it’s useful to other learners.