When you’ve installed the plugin you invoke it with a keyboard shortcut and then type in natural language commands, e.g. email this page to John, map the local area, etc. You can also highlight text on the page and ask Ubiquity to define or translate the terms (it will serve the translated text back into the page.) There are currently about forty different commands.
I feel pretty sure that Ubiquity will become, well, ubiquitous fairly soon, graduating from plugin status to become an important way we all interact with web sites and applications.