A pre-existing conditions waiver is a benefit that can be purchased either in addition to, or as part of, a travel insurance comprehensive plan. Usually, if you are in a position where you need to file a travel insurance claim due to illness of some kind, the insurance company retains the right to look back through your medical records for a defined period of time (generally this is anywhere from 60-180 days prior to the effective date of your insurance, depending on the policy and provider), to help them determine whether or not your illness resulted from a known pre-existing condition. If you have a pre-existing conditions waiver then the exclusion does not apply, which can be very good news for travelers who have chronic but well-managed medical conditions.
In order to purchase a pre-existing conditions waiver, you will generally need to meet a set of criteria outlined by the travel insurance company. Most companies stipulate that you must buy your travel insurance policy within 10-14 days of making your initial deposit on your trip in order to be eligible for the pre-existing conditions waiver. In addition, you must be medically able to travel as of the day that your travel insurance takes effect.