Explain what the index on a radical does

The quantity n written to the left is called the index.

Radicals - The symbol n√x. used to indicate a root is called a radical and is therefore read "x radical n," or "the nth root of x." In the radical symbol, the horizontal line is called the vinculum, the quantity under the vinculum is called the radicand, and the quantity n written to the left is called the index.