Content injection means that your site is also showing content that was not added by you the admin. The content that was injected can be either on your legit pages or in the form of completely new pages…but you in most cases don’t see it on the site. How come? Hackers often use various Cloaking techniques that basically show different content to human users directly loading the site in browsers from what is shown to search engines and their bots.
This means that when you load a web page in your browser all will appear to be normal, but then the Search engines will display something completely different in their search results.
Most “popular” content injection types are Pharma hacks that use your website to promote pharmaceutical sales, then various knock off brand sales (purses seems to be quite popular), porn content and even essay writing…and of course none of these have anything to do with your website.