The nuclear equation is on the microlevel. It has to do with the sub-atomic particles inside the atom. The notation used here is shown in the picture. The superscript represents the mass number (protons+neutrons), while the subscript represents the atomic number (protons only). You balance the nuclear reaction such that the sum of the subscripts and the sum of the superscripts are the same on both sides. Thus, the reaction needs one atom of deuterium (H²) and one atom of tritium (H³), to yield a Helium atom and a neutron.