The output appears in the terminal window.
You can use the Page Dn, Page Up, or arrow keys to move through the document (to exit the man page, hit the "q" key).
However, whilst it's a very useful feature, I also find reading the man pages in the terminal window a little clumsy so I was surprised and delighted, therefore, to find that the complete set of man pages for every release of Ubuntu are available online. The html format is much easier to read and each web page (or man page) has links that are much easier to follow than scrolling through a document in the terminal. There's also a search bar to speed up access to each man page rather than using the file hierarchy.
One thing that is missing is an apropos type search, but I suppose you can't have everything!