Not much. The words are synonyms as adjectives. The word chief has Latin roots and it is often used as a noun. "Main" is more likely to appear in technical usage. There are other words that carry the same idea, like "prime".

Can you give some sentences in order to give context? Either way I think I understand what you are getting at. Sometimes the word "chief" is used to mean the same thing as "main" as in the phrase "main idea" vs "chief idea". This is, however, and idiom. In standard context they mean rather different things. Hope this helps :)

Depending on the context they are very similar and could be interchangeable, if you want to convey an idea of the most important/prominent/significant thing. There would be situations where they are not interchangeable, such as the chief steward on a ship, but that is a job title/rank. As with most things, context is everything.