The phrase that has been abbreviated when they talk about an "app" is "application software".
When Casey overhears two people talking about an "app," the phrase that has been abbreviated is "application software." To arrive at this answer, it is important to understand that "app" is a common abbreviation for "application." An application refers to a software program that can be installed on electronic devices such as smartphones, tablets, or computers. It performs specific functions or provides specific services to the user. Therefore, when the two people mention an "app," they are referring to an "application software" that is likely innovative or impressive.
