"I like to use 'I Can't Believe it's Not Butter' on my toast in the morning, because sometimes when I eat breakfast, I like to be incredulous. How was breakfast? Unbelievable"

Demetri Martin is making a joke about his breakfast routine. He likes to utilize "I Can't Believe it's Not Butter" on his toast in the morning, which indicates that he is amazed that it is not butter. He is recommending that he likes to be incredulous, or hesitant, when he eats breakfast. By saying that his breakfast was "unbelievable," he is making a pun on the product name and indicating that his breakfast was so excellent that it was difficult to think. He is also suggesting that his breakfast was so excellent that it was nearly as excellent as if it were made with genuine butter.