"No Poo" means not
using a regular shampoo containing harsh sulfates to wash your hair. It also means "co-washing" - using your conditioner to wash your hair.
"Low-Poo" means using a gentle shampoo - one that does not contain sulfates, but may contain gentler ingredients, like Sulfosuccinate or Cocamidopropyl Betaine. Some examples are Burt's Bees, Giovanni, or Nature's Gate.