You may:
use the plugins for free on any number of sites, your own or your clients’; read, change, copy and share the code; use it in paid work and in your own products.
The one condition:
keep the copyright notice and the licence text with the code when you pass it on. The plugins come without a warranty.
Earlier releases:
the first releases of the SEO plugin (0.1.0 to 0.1.2) were published under FSL-1.1-MIT. GoSi Tech now also licenses them under MIT.
This page is a summary, not the licence. Where they differ, the licence applies. The full text is in the LICENSE file in the repository.