I was just trying to understand wrapper classes and then happen to see the below code. Understood everything almost but one thing I am not able to understand is; if I select one or two Account records by clicking on the checkbox (and not all account records through the topmost checkbox), then how the value of the Boolean variable "Selected" is becoming True from false for the selected account records?

See the below Visualforce code and controller class:
-----------VF------------------------------
<apex:page controller="wrapperControllerClassExample1" sidebar="false">
    <script type="text/javascript">
        function selectAllCheckboxes(obj,receivedInputID){
            var inputCheckBox = document.getElementsByTagName("input");
            for(var i=0; i<inputCheckBox.length; i++){
                if(inputCheckBox[i].id.indexOf(receivedInputID)!=-1){
                    inputCheckBox[i].checked = obj.checked;
                }
            }
        }
    </script>
    <apex:form >
        <apex:pageBlock >
            <apex:pageBlockButtons >
                <apex:commandButton value="Show Selected Accounts" action="{!processSelected}" rerender="table2"/>
            </apex:pageBlockButtons>
 
            <apex:pageblockSection title="All Accounts" collapsible="false" columns="2">
 
                <apex:pageBlockTable value="{!wrapAccountList}" var="accWrap" id="table" title="All Accounts">
                    <apex:column >
                        <apex:facet name="header">
                            <apex:inputCheckbox onclick="selectAllCheckboxes(this,'inputId')"/>
                        </apex:facet>
                        <apex:inputCheckbox value="{!accWrap.selected}" id="inputId"/>
                    </apex:column>
                    <apex:column value="{!accWrap.acc.Name}" />
                    <apex:column value="{!accWrap.acc.BillingState}" />
                    <apex:column value="{!accWrap.acc.Phone}" />
                </apex:pageBlockTable>
 
                <apex:pageBlockTable value="{!selectedAccounts}" var="c" id="table2" title="Selected Accounts">
                    <apex:column value="{!c.Name}" headerValue="Account Name"/>
                    <apex:column value="{!c.BillingState}" headerValue="Billing State"/>
                    <apex:column value="{!c.Phone}" headerValue="Phone"/>
                </apex:pageBlockTable>
 
            </apex:pageblockSection>
        </apex:pageBlock>
    </apex:form>
 
</apex:page>

------------Controller-----------------
public class wrapperControllerClassExample1 {
 
    //Our collection of the class/wrapper objects wrapAccount
    public List<wrapAccount> wrapAccountList {get; set;}
    public List<Account> selectedAccounts{get;set;}
 
    public wrapperControllerClassExample1(){
        if(wrapAccountList == null) {
            wrapAccountList = new List<wrapAccount>();
            for(Account a: [select Id, Name,BillingState, Website, Phone from Account limit 10]) {
                // As each Account is processed we create a new wrapAccount object and add it to the wrapAccountList
                wrapAccountList.add(new wrapAccount(a));
            }
        }
    }
 
    public void processSelected() {
    selectedAccounts = new List<Account>();
 
        for(wrapAccount wrapAccountObj : wrapAccountList) {
            if(wrapAccountObj.selected == true) {
                selectedAccounts.add(wrapAccountObj.acc);
            }
        }
    }
 
    // This is our wrapper/container class. In this example a wrapper class contains both the standard salesforce object Account and a Boolean value
    public class wrapAccount {
        public Account acc {get; set;}
        public Boolean selected {get; set;}
 
        public wrapAccount(Account a) {
            acc = a;
            selected = false;
        }
    }
}