interface circuit between amplifier and antenna
That's probably a matchbox, an impedance matching device. 



A: Do I need it?
Q: It depends on the antenna impedance/type you have it and the amplifier output type/impedance. If you have a 50 ohm output, 50 ohm coax cable and quarter wavelength antenna 50 ohm, then you don't need the matchbox. Especially if you don't how how to tune it, you'd better use direct connection, because a small mismatch is better than total mismatch.