Electrophoresis is an electrokinetic process that is used to separate charged particles in a fluid by using a field of electrical charge. It is often used for separation of molecules or DNA basis their size, density etc. The molecules are separated through an electric current and a gel. Electrophoresis is usually conducted in a box-like structure which has a positive charge on one end and negative on the other. The electric field around the gel causes the negatively charged molecules to move towards positive electrode and vice versa.
Since DNA fragments are negatively charged molecules they can be separated by forcing them to move towards the anode under an electric field through a medium/matrix.