pip is Python's package manager. It is not a part of OctoPrint, but OctoPrint uses it for managing plugins and also updating itself. pip likes to tell users if there's a new version of itself via a message looking like this:
You are using pip version 8.1.1, however version 9.0.1 is available.
You should consider upgrading via the 'pip install --upgrade pip' command.
This is nothing to get alarmed about and most importantly it is not an error in any way. OctoPrint will just happily continue to use the current pip version if it is not completely outdated, but it won't stop pip from printing out this message.
If you really want or need to update pip, you need to do some steps on the command line. If you are running OctoPi, connect to your instance via SSH and then run these commands:
source ~/oprint/bin/activate
pip install --upgrade pip
If you've performed a manual install of OctoPrint however, it will be something along the lines of this instead:
cd /path/to/OctoPrint
source ./venv/bin/activate
pip install --upgrade pip