The Drexel University Math Forum advises that no one "invented" the radical sign as it exists today. Instead, it evolved over time, originating from a German symbol that resembled a check mark. The first known occurrence of that symbol was in the book "Die Cross," published in 1525, by the German mathematician Christoff Rudolff.
The current version of the radical symbol is similar to the symbol for long division with a check mark at the front. The number under the radical is known as the "radicand." A small number, known as the "index," sometimes appears at the front of the radical above the check. This small number represents which root to take from the radicand. For instance, if a question asks about the cube root of 27, then 27 is the radicand and appears under the radical, while 3 is the index and is represented as a small number 3 above the check.
Another way to represent a radical is a number with a fractional exponent. For example, to represent the square root of 9, it can either be written with a 9 under a radical or a 9 raised to the power of 1/2 -- both of these mean the same thing and are functionally equivalent in mathematics.