A formulary is the complete list of prescription medications covered by a specific health plan, published by the insurance carrier. The formulary shows which drugs are covered, what tier they're in (which affects your cost), and whether any restrictions apply—like prior authorization or step therapy requirements.
Every plan has its own formulary, so a medication covered by one plan may not be covered by another, or may be in a different cost tier. Find your formulary in the Summary of Benefits that comes with your plan.