My friends talk about a Prescription Drug "Donut Hole". Can you explain what that means?

Medicare Prescription Drug, Part D covers prescription medications. Part D plans have an initial coverage period (in 20016 up to $3,310 in medication costs), which may include a deductible. Then the person would most commonly have a coverage gap or donut hole. Drug manufacturers write off a portion of the drug costs during this period so that the insured pays 45% of brand RX costs and 57% of generic costs. After the insured has paid $4,850 out of pocket, the Part D plan pays all but 5% of drug costs or a small co-pay.