The zeeman effect is the splitting of the atomic spectral line of an atom into multiple components in the presence of an external magnetic field.
This phenomenon is caused due to interaction of the magnetic field with the magnetic moment of an electron.
In an atom, when an electron shifts to a different energy level, it emit light of a specific frequency, but when an external magnetic field is applied and because of the presence of the magnetic moment in an electron, this energy shifts of an electron in energy level gets splitted into multiple energy level. And the level of split in the energy level is directly proportional to the strength of the external magnetic field.
Visual illustration of Zeeman Effect:
