It is an electrostatic attraction between opposite charges.
It is a weak force of attraction.
In a solution containing hydrocarbons one would not expect that one hydrocarbon molecule would not be attracted to its neighbors because hydrocarbons have nonpolar covalent bonds.
However, there is a weak force of attraction between these hydrocarbon molecules.
van der Waals proposed a mechanism by which these forces of attraction could arise.
The electrons that are shared by the Carbon and Hydrogen atoms in the covalent bonds do not have static positions. Their positions are best described by assigning probabilities to their positions. At certain times the protons and electrons of two hydrogens that are bonded to different hydrocarbons can be depicted by the following arrangements of charges + - + -.
Thus a hydrogen atom of one hydrocarbon would be attracted to the hydrogen atom of an adjacent hydrocarbon. This means that the hydrocarbons are attracted to each other.
These arrangements of electrons are short lived and not permenant. Thus the forces of attraction are constantly being formed and lost.