Good question, and one that has been addressed a few times. But it doesn't hurt to reiterate several points.

Janitors' job is described briefly on the Janitors page:

Editors "clean up" nodes which have been submitted for consideration (What is consideration?) or Editor Requests. They are often called "janitors" since they mostly fix ugly formatting, remove duplicates, etc. and avoid editing content.

This group was announced here.

There is a key phrase there: "avoid editing content". We enveavor to repair formatting that makes a node practically unreadable. We fix problematic node titles so that they're searchable, occasionally we obscure someone's username and password when they accidentally or naively post it as part of a code snippet included in a node, and I'm sure at some point in time a Janitor has probably removed an f-word or somesuch linguistic gem. We have at times obscured links to materials posted in a way that infringes upon a known copyright, we assist in coordinating the consideration and removal of pure duplicate nodes. We unconsider a few nodes, making an annotation that the vote tally favored "keep"ing the node as-is, we act upon sane and affirmatively voted considerations, we send people pointers to nodes that explain how to format posts, and so on. But for the most part, we try to (and hope to) do nothing to the vast majority of nodes.

We don't touch content to fix spelling errors, grammatical errors, textual ambiguity, linguistic shortcomings, logical fallacies, errant answers, arrogant attitudes, or for any other reason except for the rare cases listed above.

If you've ever posted a really messy node, and come back later to find that someone has fixed your broken HTML, that was a janitor doing his/her job.