I learned that if we are referring to money in general, then we don't use "the".
So: "I need money" = I need money in general (any money).
But when watching a movie, I heard people say "I need the money".
Here is the plot – a girl is currently out of job and she came to a job interview:
Interviewer: "Why do you want this job?"
Girl: "I need the money."
So, I think maybe her sentence is implied and its complete sentence could be:
"I need the money to support my lifestyle."
So, when should we use "I need money." and when should we instead use "I need the money."?
Best Answer
"The money" is a particular source of money
Omnidisciplinarianist's answer starts out by saying (quite rightly) that "the" is used when it is "a specific amount or parcel of money". However, the rest of their answer seems to suggest that the "specific" part is about "being specific to a purpose". I disagree; I would argue that it is about "being specific to a source".
In your example...
...you rightly observe that the girl hasn't specified what she needs the money for. However, this is not important for her choice of "the money" rather than "money". She means that she needs the money that this job would pay.
Writing this answer has gotten me thinking about this choice of phrase. It occurred to me (and you too may wonder), what's the real difference? A dollar is a dollar, or a pound is a pound. Aside from practical or legal concerns, you would never have reason to say, "I need this money, not that money," if both were the same amount.
While I have said that "the money" is used when referring to a particular source, I think it's worth pointing out that it isn't important for that money to be any different to any other money. If the girl in your example interviewed for two jobs, paying the same, then it's not true that she needs "the money" specific to one job or the other. However, she would still likely say "the money", not because she needs that specific source of money, but because she and the interviewer are talking about a specific source of money already.
In other words, the difference between these phrases...
...isn't really that you need this specific source money, but that you are already talking about this specific source of money. There is an implication that this specific source of money is the right amount for you, but that doesn't mean you would change your wording based on whether you had other alternatives available.