Magnets attract magnetic substances such as iron, nickel, cobalt etc. They can also repel :

Magnets attract magnetic substances like iron, nickel, and cobalt, which are classified as ferromagnetic materials [t1][t2]. However, magnets can also repel certain substances. Diamagnetic substances, such as bismuth, graphite, water, and copper, exhibit a negative magnetic susceptibility [t1][t4]. When placed in an external magnetic field, these materials develop an induced magnetic field in the opposite direction to the applied force, resulting in a weak repulsive force [t2][t5]. While ferromagnetism and paramagnetism involve attraction (strong and weak respectively), diamagnetism is characterized by this inherent repulsion [t2][t6]. Although diamagnetism is a fundamental property present in all matter, it is often very weak and masked by stronger magnetic effects in other materials [t5][t8]. Therefore, while magnets attract ferromagnetic and paramagnetic materials, they specifically repel diamagnetic substances.

Sources

1. [1] https://www.sciencedirect.com/topics/materials-science/magnetism
2. [2] https://personal.eee.strath.ac.uk/barry.williams/Book/Chapter%2032.pdf