the multiplier effect is just basically how one small change in the economy can lead to enormous impacts as this change is felt through the different stages of the circular flow.
for example, suppose that consumer's real income increases. This would lead to increase in consumption as they have more disposable income, which leads to higher revenue for suppliers, which leads to increased production as there is higher demand for the good, which means increased output, production etc which means economic growth.
As the increase in income of consumers travel through the circular flow, the amount gets multiplied through the multiplier effect.