Intermolecular forces are forces that keep molecules together. For example, the forces between two water molecules. The stronger the intermolecular forces are, the more "solid" is the matter going to be, meaning that the intermolecular forces are the strongest in solids and weakest in gases.
Make sure not to confuse intERmolecular forces (forces between molecules) and intRAmolecular forces (forces between atoms that make up a molecule).