The duty of disclosure is related to material information but slightly broader. It's not just about answering the questions on an application form accurately. It's an ongoing obligation to tell your insurer about anything that might affect your cover.
In practice for pet insurance this means being honest and complete when you apply, declaring any pre-existing conditions or relevant health history, but also telling your insurer if anything changes during the policy that might be relevant. If your pet is diagnosed with something new, if you move to a different area, if you start using your pet for something different than what you declared — these might all be things your insurer needs to know.
The duty of disclosure also applies at renewal. When your policy comes up for renewal you're essentially entering into a new contract, and if your pet's circumstances have changed significantly since the last application, you should make sure that's reflected in the new policy.
What happens if you breach your duty of disclosure depends on whether it was deliberate or accidental and how significant the undisclosed information was. At the serious end, an insurer can avoid the policy entirely, meaning treat it as if it never existed, and refuse to pay any claims. For less serious cases they might adjust your claim payment or add an exclusion going forward.
The safest position is always to disclose more rather than less and let the insurer decide what's relevant. Their job is to assess the information. Your job is to provide it accurately.