When we say "money is tight", it means we have very little of it (hardly enough for our daily needs). "To mention" means "to talk about" or "bring into a conversation". "I have very little money. So little that it's not worth talking about." That's the way it's... simply read. :D

Mi español no es muy bueno pero se significa que no tengo muchas monedas y no le gusta hablar de eso.