Phrase:
お [飲]{の}み[物]{もの} は [何]{なに} が いい です か
O nomimono wa nani ga ii desu ka?
What would you like to drink?
What I would like to know is what いい です (ii des) does for the sentence here? I've read that いい can translate to "good", but I don't see how that fits here.
I'm very new to Japanese and my immediate reaction is to think this might translate to "What beverage is good?"
Well, your reaction is not that wrong actually. You just need to think about the context in which the sentence is being asked.
いい means indeed good and here the topic of the sentence are the drinks, marked by the particle は. So, a very literal translation could be: "talking about drinks, what is good?".
This construction with なにがいいですか is often used to ask someone what he would like to drink/eat etc. You can see it in English as if you added "for you" at the end of the question. Something like "What drink is good for you?". In a much broader sense, "What can I get you?".
In English of course you would hardly say "What drinks would be good?", hence the translation becomes more naturally "What would you like to drink?".
You are correct with "What beverages are good?" in terms of word-for-word translation. However, いい (ii) and いいですか (ii desu ka) have different meanings.
いい (ii) = good
～いいですか (ii desu ka) = Can I ~ , May I ~ (if you use the polity form), would you like ~, are you okay with ~
If we directly translate "What do you like to drink?", it will become "なにをのみたいですか?". However, this isn't exactly polite.
* verb + tai = desire (e.g. のみたい=desire to drink, たべたい = desire to eat, みたい = desire to watch)
** Depending on the particle used in front, the meaning can change a little (ref: Noun + で(も）いい VS +がいい (=Noun + de(mo) ii vs + ga ii))
