Answer:-
The protons of every nucleus spins inside the nucleus. In hydrogen also they spin. When the hydrogen is present as hydrogen gas, then there are two hydrogen atoms, each having 1 proton.
It may happen that the spins of both the protons are in the same direction. Then this type of hydrogen is called ortho hydrogen.
If the spins are in opposite direction, then it is called para hydrogen.
Ordinary hydrogen gas is an equilibrium mixture of ortho and para hydrogen
