Average Propensity to Save is across the entire income . So if you earn 50 000 a year and you save 10 000 then your APS is 20%.
Marginal Propensity to Save refers to the amount that would be saved if you were given an extra dollar. So for example if your income was 50 000 and I gave you one more dollar so that it was 50 001, you might save 50c of that extra dollar so your MPC would be 50%.
The reason a distinction is made is because the two measures will not necessarily be equal (as demonstrated in this example). the APC for 50 000 will be approximately 20% (slightly more), but the MPC will be 50%. The reason for it is because the more you earn usually the less you spend for every dollar you earn. So the first 10 000 will have a high MPC, and the last 10 000 will have a low MPC. They average out to form the APC