How much a household spends depends on how much they have to spend. Disposable income is the income actually available for spending and saving. The relationship between consumption and income is relatively stable. As real income goes up, real consumption spending goes up. The consumption function the relationship between the level of income in an economy and the amount households spend on consumption, ceteris paribus.
Disposable income is considered the independent variable and consumption is the dependent variable.
Consumption depends on disposable income. There is a positive relationship between the level of disposable income in the economy and the amount spent on consumption. It equals the change in consumption divided by the change in income. It equals the change in savings divided by the change in income. Along a given consumption function, consumer spending depends on the level of disposable income in the economy, ceteris paribus.