What does the number 2 h2o mean?

H20 is a chemical formula. H stands for Hydrogen, O stands for Oxygen. An Oxygen atom has a different makeup than a Hydrogen atom. So H20 means there are 2 hydrogen atoms and 1 Oxygen atom. H20 is essentially H201, but the 1 is silent. Atoms make up everything.. Every substance.. etc. You'll learn about it in 8th grade.