What is the difference between 2H and H₂?

H₂ is molecular hydrogen. It is a molecule of hydrogen that consists of two hydrogen atoms bonded together by one single bond. A subscript also denotes how many of a particular atom is in a molecule, thus H₂ has two hydrogens.

2H denotes two moles of elemental hydrogen.

(It should be noted that elemental hydrogen is not bonded to anything.)