Marginal propensity to consume (MPC) no relation Average propensity to save
A
        P
        C
        =
        
          
            C
            Y
          
        
        =
        
          
            
              C
              
                a
              
            
            Y
          
        
        +
        c
      
    
    {\displaystyle APC={\frac {C}{Y}}={\frac {C_{a}}{Y}}+c}
  
It can be also explained as spending on every monetary unit of income. 
Moreover Keynes´s theory claims that wealthier people spend less of their income on consumption than less wealthy people. This is caused by autonomous consumption as everyone needs to eat and get dressed, so they buy a certain amount of food and clothes or pay rent, they all spend some amount of money on these necessities. So the ratio is falling with higher income and wealth. This is why it seems like the poor consume more than the rich. But they only need to spent larger amount of their income on consumption because they have less money available.
Average propensity to consume is not as significant as the marginal propensity to consume (MPC) which represents an additional change in consumer spending as a result of an additional change in household income per monetary unit  and it is calculated as derivative of consumption function with respect to income (ratio of change in consumption to change in income). It is used for calculating multiplier in aggregate expenditures model.Characteristics of APC
Average propensity to consume is decreasing
Since autonomous consumption in positive (Ca>0), the ratio of APC falls with increase in disposable income because with increase in income the proportion of consumption expenditures is decreasing as it creates smaller part of income. Income also rises faster than consumption. Since Keynes considered savings a luxury, he expected the rich to save more from their income than the poor.
no relation