The adjective cumulative describes the total amount of something when it's all added together. Eating a single chocolate doughnut is fine, but the cumulative effect of eating them all day is that you'll probably feel sick.
The origin of cumulative is helpful in remembering the meaning; it comes from the Latin cumulatus for "to heap." If something is cumulative, it is heaped together so it can be counted up to get a total number. The cumulative snowfall for the whole winter isn't just the amount of snow that fell in one month, but rather the number of inches that fell every month that winter to get the total, cumulative, amount.