You can think of it as "You shouldn't talk about a banquet before a starving man". Some variations of this saying has it as "주린 귀신", where 주린 means 굶주린 (starved). So 귀신(ghost, spirit) here means a ghost of someone who died of starvation. The saying describes how one can be in a bad spot by saying the wrong thing to someone very keen on something. Proverbs can be taken different ways, so this one might be considered an admonition not to be insensitive to others. But I'd rather think it's relating the bad experiences people had from saying something inadvertently. Think of how some lottery winners discover they have so many relatives and friends they never thought they did. So an apt English version of it might be "Never advertise that you have won the lottery".

여기서 귀신이란 죽은 조상들을 뜻하고, 떡은 제삿상에 필수로 올라가는 음식이기에 나오는 겁니다. 즉, 귀신이 듣는데 떡 얘기를 하면 제사철인줄 착각하여 웃지못할 일이 벌어질 수 있으니 조심하라는 예를 드는 것으로, 듣는 사람 입장을 고려하여 발언에 신중을 가하라는 뜻이고, 혹은 특정인에게 혼선을 줄 수 있는 얘기라면 그 사람들이 듣지 못하게끔 주의하라는 의미입니다.

I don't even know what this means. hehe.. I am not sure if "떡" means korean rice cake or something sounds like "떡". it may depend on the context which you were in. if you were eating korean rice cake then, it be it. I guess it sounds like "호랑이도 제말하면 온다". it can translate "speak of the devil" ? ... I just googled it now hehe.. now I know what it means. :) "떡" means "korean rice cake"(which sounds awkward for me but many people say like that.) if you talk about "떡" before someone who really likes it, how would he react? yeah, you can use this expression in case when somethings are issued in the talks you are talking and a people who really like the issue are about to speak with so much passion in him. it's kind of addroable expression when you want to express the fact that the man/woman who you are talking with are/have such fancy about it. I don't know what fancy is but I just feel like it. for instance) suppose you are with your friend who is big fan of like Disney Land. you accidently talk about Disney Land, you can notice a glow on friend's eyes. That's when you can use this expression. It means I talked about it by mistake. but it's just kind of kidding.