The book Pro Git says that the staging area is just a list, or index, that says which files will be committed when a git commit is done, and now the name index is more commonly known as the "staging area".
But if we modify the file foo.txt that is already part of the repo, and use git add foo.txt to stage it, and modify the file again, now the file is both "staged" and "modified" (as seen in git status), and if we commit, the "staged" version will go into the commit. The second edit won't go in.
So how can the "staging area" keep track of what the first edit was if it is just an index -- a list of files?
Index is a view of your working directory that is ready for commit. It can be seen as a pre-commit state and is not as simple as a "list of files". When you do git add, the file (with the change) is added to the index and the newer changes will not be see until you add them too.