RSS is a method of subscribing to content from websites, viewing updates from various sites collected in one place via an RSS reader. More info about it here:
Many gaming websites, blogs, podcasts, etc (including BGG) supply RSS feeds. If you use an RSS reader, you can subscribe to RSS feeds of various BGG content.
E.g., to see all contributions by a given user, you can subscribe to an RSS feed for that user. On every user's page, on the Contributions section, there's a link to the user's RSS feed. It's on the orange icon next to the text "Contributions by USERNAME".
To see changes to a particular BGG wiki page, you can subscribe to an RSS feed for that page by clicking the orange icon to the right of the wiki page title.
If you are a BGG Supporter at the $25 level or higher, you can access an RSS feed of your subscriptions feed (according to this forum comment.)