Content generation or content creation means creating some kind of media for other people's consumption. So, for us freelance writers, this generally means creating text-based media for readers to consume. Since "content creation" and "content generation" is generally referring to digital media, we're talking about web content here: articles, lists and other kinds of text creation or generation.
For freelance writers, this is a specific form/niche in biz-tech freelance writing. The content was generally thought of as blocks of information for websites that is used to capture search engine attention while also providing information to the reader. However, recent developments on the ever-changing Internet now includes content created for content marketing purposes (known as content marketing), another popular niche with freelance writers.
Content, or web content, needs to be written by someone, and website owners often turn to freelance writers to produce this content. Before content marketing and writing for the web developed and coalesced, content generation caused some controversy in the freelance writing world as it generally meant low pay, high-volume work aimed at search engines (not readers) through third parties known as content mills. In addition, many writers get burned out writing content (or at least the high-volume kind). For these reasons, many writers got burned out quickly and moved past content generation writing ASAP.
Creating web content does have its advantages, though. For example, it is readily available and in high demand. In addition, as evidenced with the onset of content marketing, it is a steady and adaptive niche to be in! Having a knowledge of SEO and the kind of writing that web readers prefer will help you in this area.