Light rays coming from a long distance parallel to the axis of the concave mirror, are reflected towards the focus of the concave mirror along its axis. The energy of the light waves (electromagnetic waves) is concentrated at the focus of the mirror.
Similarly, the telecommunication signals are also electromagnetic waves like light rays. They are incident on the dish antenna from the satellites or other dish antennas placed at long distances. They are mounted and adjusted so that the axis of the dish antenna points to the satellite or other dish antenna.
Hence, the electromagnetic signals of TV are incident parallel to the axis and get reflected to the focus. At the focal point of the dish, there is an electronic circuit kept, which absorbs the electromagnetic energy and generates voltage / current. These voltage / current signals are decoded into image, voice and data signals.