Each IV (Individual Value) give's a point's boost to the Pokemon's stat.
You have an Arcanine with 100 HP and it has no IVs in HP. But if it has 20 IVs in HP, Arcanine will have 120 HP instead. The max number IVs a Pokemon can gain is 31, so if you max your Arcanine's HP IVs you'll have it at 131 instead of 100, which is a tremendous boost.
Hope I helped!