Ghostery is a browser extension, which is a piece of software that extends the functionality of a web browser on a desktop computer. Ghostery monitors all the different web servers being called from a particular web page and matches those with a library of data collection tools (trackers). If it finds a match, Ghostery shows that tracker within a list in the purple box information bubble or summary panel in the extension. If you've configured Ghostery to block communication with one or more of these companies, it prevents the call from leaving the browser.
This is different than opting-out or blocking cookies, because those strategies still allow the browser to communicate with the web server (they just try to alter the communication or the response). When blocking is enabled, Ghostery never allows for communication in the first place.