Cats pant with their tongues out when they're hot. Cats have sweat glands on the soles of their paws, so they usually cool down by panting and sticking out their tongues. When the temperature is too high, a cat's body temperature also rises. Using only their paws to dissipate heat is insufficient, so they pant with their tongues out to release a large amount of heat from their bodies, thus cooling down.
If your cat is still panting with its tongue sticking out even when the weather is cool, it may be experiencing a stress reaction. This usually means the cat is too nervous or has been frightened. In this case, you can hold the cat in your arms and gently stroke its fur to help relieve its tension.
