Cancer kills in a number of different ways, depending on the areas of the body infected, but findamentally it destroys healthy body cells until it destroys enough cells in vital organs such as lungs, kidneys, blood, bones, etc., that the body can no longer function, and it dies.
I am sorry to put it so brutally, but there is no easy way of describing such a terrible condition.
My deepest sympathy to you and your family about your dad, I do hope he will not suffer for long.
You must cling on to the thought that the pain of his passing for you, will be the release from the pain for him, and peace at last.