To have your back against the wall is to a) recognize that you're in a situation where b) you must act in a specific way to either gain a desired outcome or avoid an undesirable one.
A clear example that many will be familiar with is school exams:
You have an exam tomorrow that you have not studied for. Your back is against the wall — either you study for it to increase your chances of doing well, or you don’t and you fail.
Situations like this where the consequences are fairly obvious aren’t the ones we should be most worried about, though.
We should be more concerned about situations where our back is against a wall, but we don’t realize it and therefore don’t act.
Such is the case whenever we look back in time and experience regret. Regret from not doing more, from not doing something specific, from not taking a leap.
As a thought experiment, imagine you’re sitting where you are one year from now and ask yourself: what will I have been proud of doing? What would I regret not having done?
These represent the very wall that your back is against right now.
The urgency is as real as it gets.
You have one year to do them, after which the consequences of what you do or don’t do will be felt in very much the same way you can feel joy and regret right now about the things you did and didn’t do last year.