If you are not a programmer you will not need this. But if you are interested in learning programming, you would need to start learning the basics of coding first.
custom_json is a simply a container where you can store text in Hive blocks, similar to posts and comments. But it is not visible to everybody unless you build an app to display the data stored in them.
Yes, I am not a programmer, but my interest in both crypto and education made me curious about codes. I guess learning programming might be too late for me. Anyhow, I am thankful for explaining such programming jargon in simple terms.