What does it "keep all microcontroller resources busy" mean in general terms?

Is it hardware related like microcontroller ? Is it peripheral related? Is it program related?

 

A microcontroller has multiple resources: memory, CPU, I/O ports, possibly ADC and/or DAC, temperature sensor, etc. It would take a program, or combination of programs, to cause all resources to be busy.

 

You are probably going to need to provide some context of where you read it.

But your typical micro can be tied up by putting large code (or a bug) in an interrupt routine.

This bug would ruin an Arduino's day...

ISR(TIMER1_COMPA_vect){ 
    for(int i = 1; i = 1; ... ){
           ...
        }
    }
}