recordSetVar in salesforce

 recordSetVar :The attribute recordSetVar indicates collection of records.This variable can be used to access data in the collection.This attribute is only used with the standard controller.
For example recordSetVar="Accounts" means this variable contains list of account records.
Example:
<apex:page standardController="Account" recordSetVar="Accounts">
    <apex:pageBlock >
        <apex:pageBlockTable value="{!accounts}" var="a">
            <apex:column value="{!a.name}"/>
            <apex:column value="{!a.phone}"/>
            <apex:column value="{!a.accountsource}"/>
        </apex:pageBlockTable>
    </apex:pageBlock>
</apex:page>
Output: