The interest rate function is a mathematical representation that describes how the interest rate varies over time, often used to model the accumulation of interest on investments or loans. This function is significant because it helps in understanding the relationship between the principal amount, time, and the total interest accrued, and it plays a crucial role in evaluating financial growth through integration. Furthermore, this concept connects with the net change theorem, as it allows for calculating the total change in value over a specified period based on varying rates of interest.