AWS CLI stands for Amazon Web Services Command Line Interface. It's a really awesome tool because it grants you access to interact with all your AWS services directly from the command line. Thus saving time from logging onto the AWS console which in turn makes you more time efficient. If you have not used Amazon Web Services or you don't have the AWS CLI installed on your machine then you need to head over to the getting setup guide for specific steps to getting setup correctly.
Ready to learn AWS CLI for s3? s3-basic commands.