Azimuthal radiation pattern of a typical directional antenna, showing main lobe, back lobe and side lobes.  The distance from the origin shows the signal strength (power density) as a function of angle.  Due to interference between radio waves emitted by different parts of the antenna, a typical antenna radiation pattern has lobes, directions in which the radiation strength is a local maximum, separated by nulls, directions in which radiation is a local minimum or zero.  In a directional antenna the largest lobe, showing the direction of most of the radiation, is called the main lobe, which represents the desired direction of the radiation.  The rest of the power is radiated in sidelobes, usually representing unwanted radiation in other directions.  Many antennas emit a large signal in a direction opposite the main lobe; this is called the backlobe.
Own work
Timothy Truckle