There are numerous applications of integrals in Calculus 2. One of them that will help you understand this aspect is an application of Volterra integral equation. As you will learn, these must be divided into two groups where you have the first kind and then the second kind. You can take a look at the questions dealing with integrals that will also refer to Fredholm integral equation with the fixed constant. You should take a look at several answers and seek a detailed explanation that will explain the role of the unknown function as well.