Triage

An easy example comes from medicine where the word is used most often, a doctor in field hospital with 20 patients, they need to triage whom to treat first and hope the rest can survive until they're done with the ones they've chosen to treat, or decide that some cases are simply too far gone that they'd need so much more care than he'd be able to give without severely detrimenting the treatment of the other patients (and possibly instead of few lost cases, he'd have lost all but the one they focused on).

Another example would be that you have only one or two squads in your command and you have to deal with 5 attacks by the enemy across different locations.

Generally this implies that the problems don't sit idly by as you handle the other problems, they go progressively worse as time passes until finally leaving you with little to nothing to work on after sufficient time has passed (you were too late to do what you could've done). This not a scheduling challenge on how to handle all cases most efficiently, this is about the need to let some things go unhandled.