Hey hi hello! This is definitely one of those things that I'd design a bit differently nowadays, or at least present better in the text. The whole game is definitely designed with a bit too much "inside baseball"/assumptions made about peoples' familiarity with mechanics and it ends up asking for more lifting than I'd like.
That said, the explanations are there, they're just a bit tucked away on like a single page and then somewhere separate- so it's a bit hard to follow. Success & failure are basically you either pay the cost or don't, and do the thing or don't accordingly. Major success is a "crit" with a pair of aces, and there are some extra benefits you get as a result.
Mixed Successes only come from Bargains- if you don't want to, or can't, succeed a Check the Maestro can instead make an offer for a middle-ground; you can give me something else, and in exchange you won't totally fail the check.
It's all very much designed to ask the Maestro to make some decisions based on what they think feels best, and to be interpreted.
I hope this helps! Last thing I'd say is our Discord has a spot to talk about G&S, so you could totally swing by and chat mechanics and stuff in there~ https://bytes.rip/discord