Underwriting? (plan, Medicare, care, cost)

[dkf747]

I see discussion from time to time about Medi-Gap plans. Sometimes people mention Underwriting may be necessary (SSDI recipient under 65). Can anyone describe exactly what is,or what it might cost? You can also link to a site that explains in in detail if you don’t want to explain it yourself. It’s something I need to understand now that I have Medicare part B. Thanks in advance.