Dispersion force occurs when electrons are distributed in such a way that they temporarily make an atom form temporary dipoles. For example, the electrons may be bunched up somewhere causing that "side" to become slightly negative while the other "side" becomes oh so slightly positive. This enables the temporary dipole-dipole interactions between adjacent molecules, and thus forms an intermolecular force between them. This IMF requires energy to be input into the bond to break it and thus results in an increased boiling point depending on the extensivness of the dispersion forces between molecules. That is why longer hydrocarbons have a higher boiling point than smaller ones, because the longer non-polar alkyl chains can participate in more extensive dispersion bonding between molecules.
Feel free to correct me or add anything! The more and better information, the greater!
