The Animal Health Certificate is the document GB residents use instead of an EU pet passport, since Great Britain no longer issues them. If you're travelling from an EU country and already have a valid EU-issued pet passport, you don't need an AHC at all — your passport is still accepted for entry into Great Britain. The AHC is only for travellers without one. It's issued by an EU-accredited veterinarian no more than 10 days before the trip, and confirms the microchip, rabies vaccination, and — for dogs — tapeworm treatment are all in order.
Its validity has two different numbers worth keeping straight. Once issued, the AHC is valid for 4 months of onward travel within the EU and for re-entry back into Great Britain within that window. But if you're travelling from GB into the EU, the certificate is only valid for entry into the EU for 10 days from the date it's issued — so a certificate that still has months left on it for getting you home can already be useless for getting you in.
Because it has to be issued within 10 days of travel, this is one appointment that genuinely cannot be booked too early. Most vets recommend booking the AHC slot as soon as your travel date is fixed, since EU-accredited vets who can issue it are a smaller pool than general practice vets, and the appointment windows around peak travel periods fill up.