Intermolecular forces can be defined as the attraction between molecules. This is not the actual bond but a force that exists between molecules. For example in a chlorine molecule which contains 2 chlorine atoms bonded by a covalent bond. The bond itself is a covalent bond which is a shared pair of electrons. However if we have 2 molecules of chlorine between the two chlorines a weak attraction will exist, this is an intermolecular force. Note that it is not called a bond as it is just an attraction, and being weak it can be broken and made again very easily. For example if we had hydrochloric acid this contains a compound of hydrogen bonded to a chlorine atom. But between these covalently bonded compounds a weak intermolecular force exists.
This intermolecular force can be bigger if the molecules is longer. For example when we are separating crude oil using distillation we have many different fractions (parts) of the crude oil mixture and we heat them up. Each one of these different fractions requires a different amount of energy. This is because longer molecules have more intermolecular forces between them, so they need more energy to break these intermolecular forces to turn them into a gas, meaning they have a higher boiling point.
This can be seen in the example of methane and octane. Methane is a molecule found in crude oil that is one carbon long and has 4 hydrogens, this is a very small molecule. Octane however has 8 carbons and also 18 hydrogens so is a much larger molecule. As a result Octane will have a lot more intermolecular forces between its molecules meaning it needs more energy to get it to a evaporate so it has a higher boiling point.
The term intermolecular describes the space between molecules
Intermolecular forces are temporary interactions between ions, atoms or compounds that are not considered to be sharing electrons.
A covalent bond forms when two or more atoms share a pair of electrons to form a bond
Boiling point is the temperature at which a liquid turns into a gas. The particles have enough internal energy to overcome the bonds in the liquid.