Dheeraj - A wrapper or container class is a class, data structure, or an abstract data type whose instances are collections of other objects.It is a custom object defined by Salesforce developer where he defines the properties of the wrapper class. Within Apex & Visualforce this can be extremely helpful to achieve many business scenarios
Check this link for more details with example - https://www.minddigital.com/what-is-wrapper-class-and-how-to-use-it-in-salesforce/