What are "signed" cookies in connect/expressjs?

I am trying to figure out what "signed cookies" actually are. There isn't much on the net, and if I try this:

app.use(express.cookieParser('A secret'));

But still... Cookies are still 100% normal on the browser, and I don't really know what "signed" is here (I was sort of hoping to "see" some weirdness on the client, something like the data encrypted using "A secret" as salt?)

The documentation says (https://github.com/expressjs/cookie-parser):

Parse Cookie header and populate req.cookies with an object keyed by the cookie names. Optionally you may enabled signed cookie support by passing a secret string, which assigns req.secret so it may be used by other middleware.

Does anybody know?

Merc.