Kubernetes Horizontal Pod Autoscaler (HPA) is a feature that allows you to automatically scale the number of replicas of an application based on CPU utilization, achieving cost savings for workloads that experience regular changes in demand. HPA works by monitoring pod resource usage and making adjustments as necessary to maintain a target level of utilization. To use HPA, you need to define how many replicas should run at any given time using the MIN and MAX values, configure resource requests for all pods, and expose a service that can be called to increase the load. HPA is useful for scaling stateless applications and can be used in combination with cluster autoscaling to reduce costs. However, it may require architecting your application with a scale-out in mind and may not always keep up with unexpected demand peaks.