A molecule is a group of atoms bound together. It’s the next level of chemical complexity. Molecules represent the basic unit of a chemical compound, and this another of those essential basic chemistry concepts.
That’s basically it. An example is a water molecule, which is made from two atoms of hydrogen (H) and one atom of oxygen (O), held together via covalent bonds.
Further definitions include, that they are the simplest fundamental unit that can take part in a chemical reaction, and that they are electrically neutral. This latter point distinguishes molecules from ions, another type of chemical compound.