Tail insurance allows the insured to report claims that are not yet made while the claims-made policy is in force (since claims-made responds when the claim is made as opposed to when the incident occurs).  Tail is the mechanism that allows you to report claims to your insurance company after your claims-made policy expires.

When your claims-made policy expires, you will renew with prior acts or you will need tail coverage to continue to be protected.

Tail insurance can be purchased by policy endorsement or as a standalone tail policy.  MEDPLI specializes in providing standalone tail coverage to physicians when they are switching jobs (Tail insurance for doctors changing jobs), or making other changes that require tail coverage.