When a scriptwriter is trying to get a film to be comissioned, they have to write a synopsis (as in, a smaller description of the film)
What is also taught is the film to have a basic one sentence tag line, which should be the central 'essence' of the film.
So there are three levels of looking at it, the script, the synopsis, and tagline. Funnly enough, it is the tagline that gives us the real meaning behind the film, even though it's the shortest description! If the film can't be broken down to this one strong fundamental idea, then the film is probably a mess.
I think the same holds true for many great songs.
As an example, Apocalypse Now (I've made this up to give you the general idea)
Whole thing: Apocalypse Now - the script
Synopsis: A Vietnam vet is asked to go back to the war to oust an officer who has gone mad in the jungle and etc....
Tagline: Good and evil are relative.
The thing here is, lots of films have that same tagline, but they have been told in a different way.
There are lots of war films. But there's only one Apocalypse Now. It said 'Good and evil are relative' in a fantastic way - it also brought in many other elements, but because its basic idea was so strong and never forgotten they only helped to enhance the film, everything about the film says 'Good and evil are relative' and at the end it says it louder and clearer.
Many bad films 'other elements' become the film itself, and the film is never quite as satisfying - you know, bad films you watch that 'lose their way' somehow, or 'lose their direction' or 'vision'. Films that DON'T hold your attention, or are confusing, or trying to say too much.
Films with a strong 'truth' behind them such as Apocalypse Now are believable. Without this fundamental truth there is less suspension of disbelief, and a flimsy emotional connection with the viewer.
Songwriting usually deals with far simpler structures - but therein lies its power. How could you possibly write a song in only eight lines that sums up and says the amount that is in a film like Apocalypse Now?
Find another way of saying 'Good and evil are relative', and do it in a style that you are comfortable with. That's how. The song could be about a prostitute, a thief, a drug baron, school bullies, revenge, childhood trauma....lots of things.
I try and look at all my songs and see what their tagline would be. Songs tend to have simpler taglines than films. Hate, love, loss, hope, struggle against hardship, etc Some famous songs ARE the tagline, and some would argue that that's what the song title should be and what you're singing in the chorus!
As a literal example 'You can't always get what you want' comes to mind. However, there are a thousand songs you could write about that topic.
When I'm having trouble with a song that 'doesn't know what it is yet' I ask myself these kinds of questions, what is the idea behind this song - and often, thinking like that forces a NEW idea on some words that until that point meant something very vague.
A song has lyrics, is about something, and says something.