What is a cross reference?

I have been looking for the answer to this for a long time now and the only thing i can come up with is, its alot to do with codes? finding something quickily?

If i am wrong someone please do help as i am very unsure.

A cross reference is a list of things or items that will replace or do the same thing as a known item. In cars, a fan belt by one manufacture will have a number on it. That number has been cross reference with other belts by different suppliers. Many parts are this way. Ford has a factory supplied list of the parts it uses. There may be 6 different suppliers that make a part that will do the same thing.

It can be information also. That way you can look something up from several different directions. If you were looking for a Library, you could try, books, book storage, information storage, warehouse for written information lots of things that would bring you back to Library.

A family tree is a cross reference. If you had a complete one, your great aunt Harriet would lead to you.