What does the "-U" option stand for in pip install -U
Type pip install -h to list help:
-U, --upgrade Upgrade all packages to the newest available version
So, if you already have a package installed, it will upgrade the package for you. Without the -U switch it'll tell you the package is already installed and exit.
Each pip subcommand has its own help listing. pip -h shows you overall help, and pip [subcommand] -h gives you help for that sub command, such as install.
You can also find the full reference documentation online; the General Options section covers switches available for every pip subcommand, while each subcommand has a separate Options section to cover subcommand-specific switches; see the pip install options section, for example.