Steuard is licensed under the GNU General Public License version 2 (GPLv2).
The plugin inherits this license from WordPress itself, which is also distributed under the GPL. That means you are free to use, modify, and adapt the plugin's code for your own sites under the terms of the GPLv2.
It's worth separating two things. The GPLv2 license covers the software and your freedoms to use and change it. A Steuard purchase, by contrast, is what gives you access to automatic updates and support for a licensed period. The code stays GPL, but the update and support service is tied to an active license from Artifex Routing Co. See What does my Steuard license include?.
You can read the full GNU GPL version 2 license on the GNU website.
