Imagine a single spot in space that can transmit RF power equally in all directions. This is an isotropic radiator. A common aerial is a dipole, which is two wires end to end with the signal in or out taken at the two connections in the centre. When the overall length is exactly 1/2 of the wave length used, then its called a dipole. This transmits and receives signals at right angles to the centre all the way around, but not in line with its ends. Its like a dough nut around the aerial. Because of the high frequency a dipole becomes very small, so a parabolic dish is used instead. This focusses the incoming and outgoing RF collected over its total area to a dipole at its centre. This gives it its "gain" over the basic dipole. The aerial is the familiar satellite dish and acts like an optical lens.
Frank