The capital letters in chemical formulas stand for chemical elements, and the subscript number after the element symbol tells you how many atom(s) of that element are in the molecule.
For instance, one molecule of water, or H₂O, is made of 2 hydrogen atoms and 1 oxygen atom. The ₂ after the H means there are 2 hydrogen atoms. If there is no number following an element symbol—like O in H₂O—that means there is only 1 atom of that element.
Here’s a model an H₂O molecule:
Note: We can’t see actual molecules and atoms with the naked eye. One droplet of water is made up of trillions and trillions of H₂O molecules.