Computer-generated imagery (CGI) is a term used in the world of computer graphics, more specifically 3D computer graphics and special effects. Computer generated images get created by computer software.
Computer generated imagery is used in many media such as movies, commercials, television programs, printed media and in video games. Computer generated imagery is being used for visual effects because you can control almost everything you want to see in a particular 3D environment. If something goes wrong in the proces it can be changed relatively easy (with the click of a mouse button). It allows 3D artists to create images that would not be feasible in any other technology, such as huge fantasy landscapes and creatures that come straight out of a comic book.
Another huge benefit of cgi is that individual artists and developers are able to make content without the use of actors, models or expensive to make set pieces. All can (in potential) be realized within the cgi software.
The world of computer graphics is constantly advancing due to increasing computer power and new software that is being developed. For example if you watch Jar Jar Binks from Star Wars: The Phantom Menace (1999) and when you see Gollum in the Lord of the Rings: The Two Towers(2002) some big advancements can be seen. When you look closely you can see big advancements in realism and depth of the cgi effects that were being used, and that was only over a minus 3 year period!