Acceleration is defined as the change in velocity (velocity is equivalent to rate of change). In practical terms an object which is moving at a given velocity can increase it's velocity with an external force being applied (pressing an accelerator). The velocity can also be reduced with an external force being applied (pressing a brake). In our swimming pool example, if we were to increase the temperature to heat in the water in the pool, or move more water to heating elements (external forces) we can accelerate the rate of change