what is use of ActionRegion in VF page in salesforce??

Hi Saurabh,

<apex:actionRegion> is used to achieve AJAX functionality to partial page refresh.

Action region is one of the most important tags which helps in increasing page performance. So we should try to make maximum use of action region in visual force page.
actionRegion component only defines which components the server processes during a request; it does not define which of the page are re-rendered when the request completes. We will still use reRender attribute on action component to decide area which should be rerendered AJAX request completes.

One more important point to note is that even when we use <apex:actionRegion> component, the whole form is still submitted, but the only area which is inside actionRegion is processed by the server.
One more important point to note is that by passing the validation rules using Action Region.

For further, you can refer to this.

Thanks.