Answer:
A subject that you particularly want to focus on because you like it or you have a good/deep understanding of it.
Step-by-step explanation:
A good essay subject can be about anything. It could relate to reality (topics that might be discussed about right now). It would be good to choose something that you have a deep understanding of, that way it would be easier for you to write the essay. Of course, you can still pick a subject you like or want to do and if you don't have a deep understanding of it, you can always spend time researching.
(Hopefully this helps somehow)