Simple Definition:

Magnetism is a natural force that attracts certain materials (like iron or steel) towards a magnet. It is caused by the movement of electrons in atoms. For example, when you hold a magnet near a metal paperclip, the paperclip is attracted and sticks to the magnet.

Very Simple Definition:

Magnetism is a force that makes certain materials stick to magnets. For example, if you have a magnet and you put a metal paperclip close to it, the paperclip will stick to the magnet because of magnetism.