RSS stands for Really Simple Syndication - a web feed format for publishing frequent updates of site content such as news, blog posts, etc.
Publishers benefit from RSS feeds because this lets them syndicate their site content automatically. Readers benefit from web feeds by getting timely updates when something new is posted on their favorite websites.
SMF comes with a built-in RSS functionality. It can be enabled/disabled from the SMF admin area > News and Newsletters > Settings.
Once there, check/uncheck the box next to to enable/disable RSS feeds for your SMF forum.
The Maximum post length option defines how many characters from a post are displayed in the feeds. You can leave this option to the default 255 characters.
The default feed is for the 5 most recent posts:
There are also several "sub-actions" available to the above action, specifying exactly what to display. The most important sub-actions are:
Recent Posts (displays the most recent posts that you can see)
Default: Shows 5 most recent posts
limit=x - display the "x" most recent posts (if number is less than 5, it will display 5. If number is larger than 255, it will display 255).
board=y - display only the recent posts from board "y"
boards=x,y,z - display only the recent posts from the specified boards
c=x or c=x,y,z - display only the recent posts from boards in the specified category/categories
You can syndicate these links by giving your readers the URLs to your RSS feeds.
For a full list of the available actions, you can check the official SMF forum.