"A TV aerial with a properly designed BALUN (BALanced to UNbalanced device) is likely to produce much more consistent reception results. TV aerials are inherently balanced devices, such that equal voltages exist to earth from each input terminal on the driven dipole. All coaxial cable is unbalanced in nature. If an unbalanced feed such as coaxial cable is connected to the balanced aerial then currents are forced to flow down the outside braid of the coaxial cable. The currents in the braid give rise to unwanted radiation from the braid and can modify the polar radiation pattern (affecting aerial performance, front-to-back ratio, rejection etc.) of the aerial and possibly also the input Impedance of the aerial. Although DTT signals can be received using an aerial without a balun, the fitting of a balun device in the aerial is highly desirable in order to minimise the effect of impulsive interference which are known to exaggerate picture freezing and break up. The best interference free DTT reception will be obtained through the use of a properly installed roof mounted aerial and with digital quality coaxial cable and flyleads, fully screened amplifiers and outlet plates."
That's a reasonable description.
In short, a balun is a differential to single-ended conversion device (a type of transformer) that works at RF frequencies.
They can convert eitehr way round (SE to diff or Diff to SE).