FISH CLI What do the error numbers at the prompt mean?

I am just trying out the **FISH** - (the FriendlyInteractiveSHell) - CLI & whenever I type in an erroneous Command, a new Prompt appears under the faulty Command prompt with a number in square brackets. I have searched through FISH's FAQs etc., but no joy in explaining the meaning of the error-number? If I hit Enter, the prompt just reappears with the error number until I enter a random correct command, such as 'ls'.