I don’t believe “index” is a type of array, but rather a way to access a value within an array. Ramón often says “position” in the video, but FreeCodeCamp uses “index” as the technical term. The index number is a way to describe where a value can be found within an array.
For example, I can have an array of strings that lists student names:
let students = ["Ashley", "Lindsay", "Ramón"];
There’s many reasons to want to access a particular value in an array, but let’s say I want to generate a greeting for a particular student.
let greeting = "Hello, " + students + "!";
Hopefully that helps answer what you were wondering. Others can correct me if I’m wrong, I’m learning too!