Academia is the term used to describe all things related to education collectively. Everyone who has enrolled to study for anything, from kindergarten to a post-doctoral degree, has been a part of academia. It is a term encompassing the students in their classrooms, the teachers teaching them, the books they both read which are published for educational use, or the culture that has revolves around these things.
However, when people hear the word academia, that is not what comes to mind. What comes to mind is those people who make their living as college professors, researchers, etc. or are on their way to do so. These are the people who are generally called academics.
The scope of academia in the above context is practically limitless. When a person enrolls themselves into it and becomes an academic, chances are they do so to pursue a very niche field of study. You might be familiar with the popular ones like arts, science, politics, history, etc. However, there are people who invest their entire life to study one particular part of a particular field. For example, a professor in zoology might dedicate years of their life to studying the communication patterns of a particular rare animal. A person pursuing a post-doctoral degree in biochemistry might try to isolate one medicinal property of a particular plant. The kind of research that gets done like this in academia is what leads to incredible findings later on.
One of the most fundamental things about academia is that knowledge is treated as an asset in its own right. The pursuit of knowledge is not considered a practical goal, it is valued just like that – as a pursuit that needs to be pursued. A common pattern you can see among academics is that they simply follow an intellectual path that they find interesting.