A magnon in physics is a quasiparticle with magnetic properties. It is not a thing which exists on its own in free space but exists only within the structure being studied usually a crystalline solid.
Each atom in a solid has a magnetic field generated by its electrons as they spin. If these spins all align then the solid becomes magnetic at the bulk everyday level. Usually they do not align and the magnetic field of individual atoms cancel each other out.
Individual atoms can have their spin reversed by interaction with energy from elsewhere. A spin reversal itself can influence the spin of the electrons in neighbouring atoms. A wave of change in electron spins can then move through the electrons in the crystal lattice as it dissipates like waves on a pool of water. Since the electrons are very small and governed by quantum mechanics, this wave, can be treated as though it is a number of particle like entities. These are magnetic quasi particles or magnons.