"Features" add additional functionality to your site, either by adding a particular "kind" of content, or by adding back-end functionality to your site such as Google Analytics.
By default your site is set up with three "features" enabled: Publications, Classes, and Announcements. You can add many more additional features by going to the Control Panel and clicking the Features button in the top menu.
Enable additional features as you wish. You can enable a blog for your site for instance; links; a dataverse, feeds and a variety of others.
Many features also have a variety of settings which you may want to explore. Further documentation is available for each feature.