C=A+MPC*YD is actually explaining that household consumption expenditure is linearly related to disposable income. C is household consumption expenditure and A is constant, which means that consumption has an initial value, that is, necessities must be bought regardless of whether disposable income increases or not. MPC is marginal propensity to consume, which is dC/dYD from a mathematical point of view, that is, let C take the derivative of YD. The view of economics is that increasing disposable income by one dollar will increase the number of household consumption.