Matching network definition and how it improves antenna efficiency at multiple frequencies

i have read an antenna of a certain length will resonate most efficiently at 1 frequency, but it can receive radio waves at other frequencies. if you use a matching network you can increase the efficiency at other frequencies, what is a matching network? i don't know what that is.

A matching network is a network of inductors and capacitors that cancels the reactive part of the impedance seen looking into the antenna (so tuning the antenna to the desired frequency), leaving just the resistive part. The resistive part is the sum of resistance in the antenna conductors and the antenna's radiation resistance. When the antenna is used for transmitting, the power "lost" in the radiation resistance is actually radiated as radio  waves. An off-resonance antenna exhibits a large reactance in series with the small radiation resistance. A matching network cancels the large reactance, leaving just the resistance. A matching network can also transform the resistive part to a more desirable value. The ARRL Handbook (available from arrl.org) has a good discussion of RF matching networks.
Be aware that analog circuit theory and linear circuit theory approach matching networks in a different way than does RF circuit design.