what 2162 has said is pretty much what happens, but since its an explain process, you'd want some words to go with it:
1. Define the multiplier (the number of times an increase in national income exceeds the increase in aggregate demand that caused it)
2. Give a description of the process of the multiplier effect (each individual has a marginal propensity to consume, and so when they spend this proportion of it, firms receive this income, then the individuals in that firm spend the marginal propensity to consume, etc, which is in fact a limiting sum of a GP)
3. Desribe the effects of this (the increased spending allows the amount of income that flows to individuals to increase by an amount defined by the multiplier formula)
I reckon that should be enough to get the marks. If you wanted to secure the marks, maybe add in the economic growth objectives of the multiplier process.