A magnetic field is a field in which its particles are charged with electricity, causing them to point to one direction.
This field has the ability to influence materials which are sensitive to magnetism, like magnets or iron. All planets and stars, large or small, have a magnetic field. Its strength depends on the mass and materials of the object.
During a scan of Aldean DNA, Doctor Beverly Crusher was able to confirm that a failure of the magnetic field of Aldea was not responsible for the Aldeans' sterility. (TNG: "When The Bough Breaks")
Naturally-occurring magnetic fields, like the ones on planets, can interfere with sensors. (TNG: "Final Mission")
Artificial magnetic fields, better know as electromagnetic fields, can be created and manipulated to do all sorts of things like containing anti-matter. (citation needed • edit)