Simply, you put in a URL and this tool will return a web page that “translates” the page into pirate speech. The UI is so sparse here you can’t do much but put in a URL (though without knowing exactly what is going to happen).
Ideal for your talk-like-a-pirate-day browsing every September 19th. Maybe a bookmarklet that does this would be cool? Come to think of it, maybe having a browser extension that does this for you automatically on every page you visit on September 19th would be a fun little toy!