In GIT, stage is a step before commit. To stage means that the files are ready for commit.
Let say, you are working on two features in GIT. One of the features is finished and the other is not yet ready. You want to commit and leave for home in the evening. But you can commit since both of them are not fully ready. In this case you can just stage the feature that is ready and commit that part. Second feature will remain as work in progress.