Polyphagia can best be described as constant hunger. Meals can be perfectly adequate but the pet or person remains hungry, actively looking for (and eating if possible) just about anything and everything.
This is another "trick" diabetes plays on all those who suffer from it. For the most part, brain cells don't need insulin to be able to use glucose. The part of the brain which controls appetite is called the hypothalamus. It is an area of the brain which is dependent on insulin to permit it to use glucose.
Because the appetite center needs insulin to be able to use the body's glucose, it perceives the low level of insulin in the body as a lack of food or "fuel". And so it sends out signals indicating the need for more food--hunger signals causing the body to crave more food. The food eaten because of this sets up another vicious circle, as all it does is add to the excess glucose in the system.