An index might be a strange word to some people. Generally it means a list of items that all have certain properties. If you were to create a list of all the cities in the world, you would have an index of cities. If you look in the back of a book, you would find an index of subjects covered in the book. All of these indexes are lists of items that have common properties (being a city, being a subject in a book, etc).
Indexes are extremely useful things while programming. Let's say I had a list of 10,000 recipes. Let's say then that every time someone ran our program, they would type in an…