What is page reference in Salesforce?

Hi Aditya.

Page reference is a class which consist of a URL and a set of query parameter names and values and
is use to  navigate the user to a different page or Url as the result of an action method.

Example :

public PageReference returnPage() {
        // Send the user to the detail page for the new account.
        PageReference acctPage = new ApexPages.StandardController(account).view();
        acctPage.setRedirect(true);
        return acctPage;
    }