Here it is meant allegorically for the ratio that a bookmaker will give you on a bet.
Literally this word - in this use - means how much you will win if you have bet money on it. But it is quite common to use it in this way to describe that a situation does not have a good chance.
Examples: "Hey bookmaker! What are the odds on Arsenal tonight?" "It's 1.5 to 1, but I'll give you 5:1 on them winning by 2 points or more. Are you in?"
Note that even though you say that the odds are not good about a situation that is unlikely, such a situation would actually have good odds, since the bookmaker doesn't think he'll have to pay anyone winnings on it. The correct way to use the idiom would be like Elon Musk does "the odds are not in your favour" - meaning that the bookmakers don't believe in you.
This is a literal thing in sports and other things that are betted on, generally the bookmakers are right, and if the odds aren't in your favour you will probably not win.