When a cat pants, it is an indication that something is wrong, possibly life-threatening. Panting is unusual for cats so be vigilant if you notice it. Often times it is from being overheated. They could be on the verge of heat stroke. It could also be a signal for health problems such as respiratory, cardiovascular, or neurological illness. If it is heat, take measures to cool the cat down immediately. In any case, always contact your veterinarian to be sure.
Lots of love,