A wiki, in it's essence, is made to have it's pages quickly and easily changed by different users, while preserving the history of previous versions.
Although most changes in the OSM wiki are made in good faith, some people may inadvertently alter the meaning of a page (and therefore of an OSM key or tag). In order to guarantee a wiki page's integrity, one may want to be notified of a wiki page's changes.
To be notified of a wiki page's changes by email, first the email notifications must be turned on. This can be done on the settings page, by turning on the checkbox “Email me when a page or file on my watchlist is changed”, and clicking on the button “Save”.
After that, all that is needed is to add a wiki page to the user's watchlist. This is easily done by clicking on the little star on the upper right of each page. When the star turns blue, that the page will be in the user's watchlist, and therefore the s/he will be notified of further changes on that page or it's talk page.
Note that, when you receive an email notification, you won't be notified of further changes unless you actually see those changes while logged in.