CH2O is a polar molecule. It has three polar bonds that are arranged asymmetrically, thus allowing their dipole moments to add up and give the molecule an overall dipole moment.
CH2O has a central carbon atom that forms two single bonds with the two hydrogen atoms and a double bond with the oxygen atom. Since it has three substituents, according to valence shell electron pair repulsion theory, they are arranged along the same plane at 120-degree angles from each other.
Carbon is more electronegative than the hydrogen atoms to which it is bonded and attracts the bonded electrons closer to it, thus giving the hydrogen atoms a partial positive charge. Oxygen is more electronegative than carbon and attracts the bonded electron pairs of the double bond more strongly, thus giving itself a partial negative charge. Since the molecule has partial positive and negative charges, CH2O is a polar molecule.