Tail insurance protects you from liability claims after your claims-made policy expires. It's extended coverage that keeps you protected when someone files a lawsuit for work you did in the past.
This coverage only works with claims-made policies, which cover lawsuits filed while your policy is active. For example, if you're a doctor who retires in January but a patient sues you in March for a procedure you did last year, your expired claims-made policy won't help. Tail insurance would cover this delayed claim, though coverage details depend on your specific policy terms and state regulations, so discuss your situation with a licensed insurance professional.