These are like what they sound like, small areas of content that populate a web page, usually displayed in a sidebar. One of the advantages of Drupal is how easy it is to turn blocks on and off per page, rearrange them and add them to other parts of the page, besides just a sidebar.
Takeaway
Blocks are often used for Calls to Action (CTAs) to convert site visitors into leads. Try using different CTAs to see which works best.