Under the same prescription conditions, the thickness of high index lenses will be thinner than normal index lenses and will be more aesthetically pleasing when mounted on the eyeglass frames. The prerequisite for this is that your prescription is stronger and the lenses are thicker using regular index lenses.
High index lenses are eyeglass lenses that refract light more effectively than ordinary lenses. Since they bend light more, the thickness needed to refract the same prescription is less compared to ordinary lenses under the same conditions, so thinner lenses can be produced.
High index lenses are relatively expensive because they are made of materials that effectively refract light. Generally, we recommend you to use glasses with high index lenses only when your prescription is stronger. You can check how to read prescription to see what your prescription like.