What is the difference between a chemical formula and a molecular formula? I'm having trouble understanding the textbook.

The chemical formula usually is also the empirical formula, in that it is simply the simplest ratio of atoms to other atoms that all make up the compound, a molecular formula, can be empirical, but also it can be not, in other words, a molecular formula can include a ratio of atoms, from their subscripts that are not the simplest ratio of atoms.