A "feed" is a way for you to find out when there is an update to a site without having to use your browser to visit the site. This is done via syndication or subscribing to a site's feed.

Really Simple Syndication (RSS) is an XML-based format for content distribution

To subscribe to a feed you need a feed reader or aggregator, and basic ones are integrated in most browsers that will deliver updates whenever we publish something new. These feeds use the popular RSS format which you can pick up with an RSS news reader.