At TouringPlans.com, we define "renewal" as a purchase made while someone already has an active subscription for the product purchased. Renewals are available for 1, 2, or 3 years, but only the first renewal year is given the hefty renewal discount (a smaller multi-year discount is applied to years 2 and/or 3).
If your Walt Disney World subscription expires next month, buying an additional Walt Disney World subscription for your account today (or anytime before your subscription expires) is considered a renewal. (Since renewals discounted, it's generally to your advantage to renew rather than buy a fresh subscription after your old subscription expires.)
When you purchase a renewal, that new subscription is added to your existing subscription. For example, if your Walt Disney World subscription expires two weeks from now and you purchase a 1-year renewal today, you will then have access for 1 year plus 2 weeks from now.
As noted elsewhere, subscriptions do not renew automatically.