Does exactly what it says on the box.

Appears to "do what it says on the tin" (UK readers of a certain age will understand that reference.)
It adds an unobtrusive "huh?" button to XKCD.com, in the same style as the existing "|< >|" buttons, linking straight to the appropriate explain-XKCD page.
Thank you to the developer. As other reviewers have said, this saves a separate web search!

Excellent extension, saves me a google search every time