Integrating the function of acceleration yields the velocity function when using an indefinite integral, which includes a constant of integration. A definite integral calculates the change in velocity over a specific time interval, represented as the difference between velocities at two points in time. Combining these concepts, the integral from a fixed time to a variable time provides the change in velocity since the initial time. However, this discussion highlights that while the integral gives the change in speed, it does not inherently convey the direction of that change. The distinction between vector quantities and their magnitudes is crucial in understanding this relationship.