Intermolecular means operating between the molecules of a compound while intramolecular means operating within a molecule. So intermolecular forces are the forces operating between the neighbouring molecules of a compound, while intramolecular are the forces operating within a molecule, which hold the atoms together in a molecule. As we know carbon form covalent bond and covalent bonding results from the sharing of electrons. The given statement actually means that the forces of attraction operating between neighbouring molecules in a covalent compound is weak, however as the bonded atoms in a molecule are held by strong bonds, therefore forces operating within a covalent molecule (called the intramolecular forces) are strong.