@Tanmay, Object Repository is used to store element locator values in a centralized location instead of hard coding them within the scripts. We do create a property file (.properties) to store all the element locators and these property files act as an object repository in Selenium WebDriver.
In Selenium, we call objects as locators (such as ID, Name, Class Name etc). So one of the ways to create Object Repository is to place all the locators in a separate file (i.e., properties file). But the best way is to use Page Object Model. In the Page Object Model Design Pattern, each web page is represented as a class. All the objects related to a particular page of a web application are stored in a class.