What does ut mean when a compounds chemical formula does not include any numbers?

It means that the elements are a one-to-one ratio. This happens when the charges cancel out, atom to atom. For example sodium chloride aka table salt. The charge of sodium, Na, is 1+. The charge of Chlorine is 1-. The charges must cancel out on both elements and if they don't you have to find common multiples until you do. So the formula for sodium chloride is NaCl with no numbers.