Are you using
Safari 2 as your browser (comes with Mac OS X 10.4 aka Tiger)? That has a built-in RSS reader.
If not you need a standalone RSS client. I recommend
Vienna, but you can find more recommendations
here.
Then whenever you access a site that offers an RSS feed (just a link to a special page, really) you can add that site to your reader (similar to bookmarking it) and when new articles are published on the site your RSS reader will let you know. Most often you'll get a summary of the article and a link so you can read the rest if you so desire.
About RSS.