Moving magnet is an electromechanical transducer design used in turntable tonearms. The stylus cantilever moves a small permanent magnet positioned between sets of fixed wire coils, inducing a small voltage. They produce a relatively high output signal compared to moving coil designs, meaning they do not require an extra pre-amplification stage before a standard phono input.
Heard on the job
Make sure the phono stage is set to MM, otherwise the signal will clip hard if it's expecting a moving coil cartridge.