Simple Definition:

A magnet is an object that attracts certain materials, like iron and steel, [which are types of metals that can be pulled toward a magnet]. It has a north pole and a south pole, which are the two ends of the magnet. Similar poles repel each other, while opposite poles attract each other. For example, if you bring a magnet close to a paper clip, the paper clip will be attracted to the magnet and stick to it.

Very Simple Definition:

A magnet is a special object that can make some things stick to it. It has two ends, a north end, and a south end. If you bring a magnet close to a paper clip, the paper clip will stick to the magnet.