The staging area, also called the index, is a middle layer between your working files and committed history. When you run git add, you copy a file's current state into staging. A commit then records exactly what is staged, nothing more.
Git separates it so you control what each commit contains. You might edit five files but commit only two related ones. You can even stage part of a file with git add -p, splitting messy work into clean commits.
The cost is one more step to remember. Beginners forget to stage and wonder why a commit missed their edits. The payoff is deliberate, reviewable history instead of dumping every change together.
Rewriting in plainer words…
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