The diopole is the mother of all antennas and all other antennas are merely adoptions of the dipole. A dipole consists of two wires pointing away from each other and the electrical field between these wires are generating the radio waves. This is the basis of all other antennas, you need to generate an electrical field to generate radio waves and as with electrical potential you need a reference point.
As the radio waves are generated by the electrical field between the antenna and the reference, try imagine the field between the two wires in the dipole antenna, that has a quite uniform gain pattern. Now replace one of the wires with an electrical conducting plane that is not small in relation to the wavelength of the radio waves, and imagine the electrical field generated and how this would affect the gain pattern of the antenna.
The above is merely a thought experiment to get hold of the general idea of how antennas work but with a lot of simplifications and generalizations.