With the exception of the ideal isotropic antenna, which spreads the power homogeneously in all direction, real antenna have an asymmetric radiation pattern, as they "squeeze" the field along a preferred area. So, dipole antennae radiation pattern is doughnut shaped, and patch antenna transmits more strongly in one direction.
The gain of the antenna is an indication of this factor as, while the total power emitted is constant, the strength of the field is greater in some parts - the gain is the ratio between the strength of the field in the preferred direction and the field of an idea isotropic antenna, measured in dB.
