A cartridge has astylus needle that moves. Inside the cartridge are two magnets, one attached to thestylus the other to the cartridge body. When playing a record, and bumping around the topographically varying terrain of a record groove, stylus movement generates a tiny electrical signal that goes out the tonearm wires into a phono preamplifier.
Moving Magnet Cartridges have the fixed magnets attached to the stylus while the would-coil electro-magnets are mounted to the cartridge body. The fixed magnets move up/down as the stylus tracks the groove, so we call these Moving Magnet cartridges.
The fixed magnets gives a little heavier and thus slower moving stylus but rewards with lower price-points and generally higher output electrical signal which in turn allows use of lower cost phono stages.