[Ifeffit] on Tune and Detune

Dear all, I'm new to the community and I was wondering if I can ask some basic questions here. I heard the beamline scientist mention "tune" and "detune" and I saw in some materials say "detune to get rid of high order harmonics", but what is "harmonics"? and why it's necessary to detune to get rid of high order harmonics? I'd really appreciate it if we can discuss this a little bit. Thanks. Enyuan