High index lens is thinner and lighter than lens of other materials. High index lens is especially good for people who have a higher prescription. People with a prescription of +/-3.00 D or more can benefit from high index lens. If you have a prescription of more than +/-3.00 D, you can choose high index lens for your glasses lenses. But what kind of high index lens you should choose depending on your prescription. So you should go to see an optometrist to have an eye exam and the doctor will recommend you the type of high index lens that can fit you properly.
High index lenses are made for people who with strong prescription. With high index lenses, people with strong prescription can get thinner and lighter lenses. In details, for the same prescription lenses, the higher the index lenses, the thinner the lenses. But that is not mean higher index lenses are better. You shall choose it according to your prescription. You can consult optician who will tell you which index lenses fit for your prescription lenses.