The balun is a device (often a transformer) that can:
- match a balanced antenna (dipole) to an unbalanced feedline (coax)
- match an unbalanced antenna (quarter-wave vertical) to a balanced feedline
- electrically isolate the feedline
- choke common-mode currents (caused by unbalanced antennas)
Although a balun can quench common-mode currents, it doesn't necessarily transform impedances.