These plans (sometimes called "PDPs") add drug coverage to Original Medicare. There are specific times when you can sign up for these plans (Open Enrollment), or make changes to coverage you already have (Annual Election Period between Oct 15th-Dec 7th) each year.
Medicare offers prescription drug coverage to everyone with Medicare. If you decide not to join a Medicare Prescription Drug Plan (Part D) when you're first eligible, you'll likely pay a late enrollment lifetime penalty unless you have other creditable prescription drug coverage, or you get Extra Help.
Understanding your options and how to choose the right plan for you is where your insurance agent can offer excellent support.