Debt is debt. It is a deficit, minus money, arrears, in the red, a tab, on the slate [ad nauseam in dead parrot Monty Python style]. It's not a weird* concept.
It represents money that is owed to someone and has to be repaid one day. Either* that or, when one day finally comes around, the creditors seize* the assets or they forfeit* the lot and bankruptcy is declared.
It's why Juventus and Barcelona respectively had to let Ronaldo and Messi go and why Real are desperate to shift Christian Bale and his £300,000 a week wages, because their is a ceiling* as to how much lenders are willing to stump up.
The three clubs are still trying to get the not-so-really-very-super-league off the ground as they still smell the 300-odd million quid each that JP Morgan were going to stump up. While actually trying to reduce their outgoings somewhat, the three clubs still cling to a hope that someone or something will bail them out.
*Remember learning at school, "i before c except after c"? How about, "i before c except after c, except when it doesn't" instead?