Magnetism allows Kenji to magnetize people around her, up to a 4.5-meter radius of herself at will.
She can magnetize a person's entire body or specific portions. Once magnetized, the targets either attract or repel each other, depending on whether they're of the same sex or not. Men are polarized south and women are polarized north, which is represented in the anime through a blue and a red aura, respectively.
Magnetism doesn't affect the user, or targets that are outside a 4.5-meter radius within range of the user.
Kenji used magnetism both for offense and support. With the help of two allies of the same sex, Kenji could apply her Quirk to launch one of them off.
It appears that this Quirk can be used in conjunction with regular magnetism as well, since Kenji wielded a huge magnet that could affect targets of her Quirk.