Answer:
When we have something like:
It is called the n-th root of x.
Where x is called the radicand, and n is called the index.
Then the term:
is called the fourth root of 16.
And in this case, we can see that the index is 4, and the radicand is 16.
At the end, we have the question: what is the 4th root of 16?
this is:
The 4th root of 16 is equal to 2.