Cats pant with their mouths open when they are experiencing respiratory distress or overheating. Panting is a way for a cat to cool down their body temperature by releasing heat through their mouth. Panting can also be a sign of pain or discomfort, such as if a cat has a respiratory infection or asthma. In some cases, panting can also be a sign of anxiety or stress. If you notice your cat panting with their mouth open, it is important to observe their behavior and consult with a veterinarian to determine the cause and provide appropriate treatment.