It simply refers to swapping, or switching, one set of tubes for another ... or another, etc. Since different tubes have characteristics that affect the tone and musicality, etc., people like to "try on" different sounds - by researching what is compatible, then replacing the stock tubes with a (compatible) different one.
Rolling got it's name, as I understand it, from the physical action of removing a tube safely/carefully/efficiently from the socket--by gently "rolling" (or rocking) the tube, sometimes in a circular fashion, to help with the friction of the socket hanging onto the tube ... gently "rolling" or rocking (a
slight angle to the socket) the tube, while pulling up (as opposed to
only pulling straight up ... I believe it's a kinder way to treat tubes and sockets in the long run).
The process could be called "tube swapping", "switching", "changing", etc. just as well. It's a (generally optional) part of the hobby - the adventure of seeking a different sound,
the sound, or even a different "sound profile" altogether - maybe to better match a particular headphone .. for instance.