Generally: A magnet is a material that has the unique ability to attract other magnetic material. Most materials can be magnetised by aligning certain 'dipoles' in them.
In narutoverse magnetism release is a special KG which was shown to give certain shinobi the ability to control certain magnetic materials using their chakra.Each shinobi seen had some special material which they exhibited this property with; Iron sand, gold dust etc. However it was shown that chakra can be deposited on an enemy making him magnetic and so all metallic tools used would hit the foe directly.
Example users:
Third kazekage
4th kazekage
Toroi