Imagine the liver releasing glucose at about 2mg/kg per minute per person. That corresponds to a daily quantity of about 200 g of glucose for a 70 kg person. Quite a lot really.
So the insulin that you need for food, in order to provide glucose for your cells, is also for the glucose that your liver releases. Given that the liver tends to do this regularly, it therefore needs a basal as well as continuous insulin supply. As an insulin pump user you receive pre-programmed insulin delivery that the pump slowly delivers in small quantities.
And this continuous insulin delivery for your liver's glucose is called the basal rate.