An electrical dipole is just a separation between a negative and positive charge of the same magnitude. Consider that a dielectric material is exposed to an electrical field. The effect of the electric field can be two fold.
(1)It generates electrical dipoles in the material and attempts to align them in the field direction. In other terms with a field dipoles come into being that do not present without a field.
(2)It attempts to align dipoles that are previously present in the material. In other terms the material consists of electric dipoles even with no field.
Of course we also may have a mixture of both effects: The electrical field may alter the distribution of existing dipoles while attempting to align them, and it may create new dipoles additionally. The total effect of an electric field on a dielectric material is called the polarization of the material. To understand this better, let us look at the most simple example we have a single atom. We have a positively charge nucleus and the electron "Cloud". The negative charges associated with the electron cloud can be averaged in space and time. If we now apply an electric field, the centres of charge will be separated. The electron cloud will be dragged in the direction of the positive pole of the field, the nucleus to the negative one. We may visualize that the centre of the positive and negative charges q are now separated by a distance d, and thus a dipole moment p is induced, which is defined by:
It is important to under that p is a vector because d is a vector.