It means, that part would be committed when you go for a commit. It's a good feature that git provides. Say you are working on 2 features, and by the end of the day you are complete with 1 feature code changes, and are half way for the other. You can stage the changes for the 1st feature and commit those changes, still keeping all ((both the feature changes) locally.
Read the post for better explanation: What-stage-means-in-git-source-control?