The multiplier concept asserts that a change in aggregate demand within the economy will then have a greater than proportional change in national income, hence affecting economic activity. This can be see through an increase in spending by the government to stimulate economic activity. This will then contribute to someone else's income, and by the multiplier effect, they will then spend a proportion of this income defined by the MPC, further stimulating economic activity, and this process continues. As a consequence, a small change in aggregate demand can perpetuate a greater than proportional effect on both national income and economic activity. The greater amounts of national income also equips consumers with greater amounts of disposable income, and hence they are able to further spend and further increase economic activity. The converse of this process is also true; a decrease in aggregate demand will then decrease the income of a recipient, who will then spend a lesser amount, decreasing economic activity.
There you go.