reRender Functionality Using Action poller:- 

Here we have a scenario.. Increment the counter automatically based on interval time..

<apex:page Controller="Actionpollar1">

    <apex:form >

        <apex:pageBlock >

            <apex:outputText value="{!Increment}" id="Test"/>

            <apex:actionPoller action="{!IncrementValue}" interval="5" reRender="Test"/>

        </apex:pageBlock>

    </apex:form>

</apex:page>

public class Actionpollar1

{

    public String getIncrement() {

        return null;

    }

    public Integer Increment { get; set; }

     public Actionpollar1()

        {

            Increment =0;

        }

    public void IncrementValue() {

       

       Increment=Increment+1;

    }

}