The Fortune 500 is an annual list compiled and published by Fortune magazine that ranks the top 500 U.S. closely held and public corporations as ranked by their gross revenue. (Wikipedia)
Could you please explain the term "closely held"?
The Wikipedia article I linked to in the answer your question derives from links directly to a further article explaining this:
the majority of corporations are said to be closely held, privately held or close corporations, meaning that no ready market exists for the trading of shares
We're here to help, but you could at least try to find things out on your own first. This isn't so much a matter of English Learning as just looking up phrases.