IS shows the equilibrium in the goods market. In a closed economy it is when Investment=Savings (IS). Now, when output/income rises, so does savings. You have mor emoney, you spend more, and you save more too. Hence for equilibrium (Investment=Savings), investment has to be higher and interest rates lower. (As income increases, savings exceed investment, hence banks offer lower rates to entice investors, savings falls a bit, investment increases a bit, equilibrium is reached at a lower interest rate). Therefore, the IS curve is downward sloping.