It represents the scope (the lifetime) of the bean. A request scoped bean lives as long as a single HTTP request-response cycle. A view scoped bean lives as long as you’re interacting with the same JSF view by postbacks returning null/void. A session scoped bean lives as long as the established HTTP session. An application scoped bean lives as long as the web application runs.
Which scope to choose depends solely on the data (the state) the bean holds and represents. Use the request scope for simple and non-ajax forms/presentations. Use the view scope for rich ajax-enabled dynamic views (ajaxbased validation, rendering, etc). Use the session scope for client specific data, such as the logged-in user and user preferences (language, etc). Use the application scope for application wide data/constants, such as dropdown lists which are the same for everyone.
As of JSF 2.x there are 4 Bean Scopes: SessionScoped RequestScoped ApplicationScoped ViewScoped Session Scope: The session scope persists from the time that a session is established until session termination. A session terminates if the web application invokes the invalidate method on the HttpSession object, or if it times out. RequestScope: The request scope is short-lived. It starts when an HTTP request is submitted and ends after the response is sent back to the client. If you place a managed bean into request scope, a new instance is created with each request. It is worth considering request scope if you are concerned about the cost of session scope storage. ApplicationScope: The application scope persists for the entire duration of the web application. That scope is shared among all requests and all sessions. You place managed beans into the application scope if a single bean should be shared among all instances of a web application. The bean is constructed when it is first requested by any user of the application, and it stays alive until the web application is removed from the application server. ViewScope: View scope was added in JSF 2.0. A bean in view scope persists while the same JSF page is redisplayed. (The JSF specification uses the term view for a JSF page.) As soon as the user navigates to a different page, the bean goes out of scope. Choose the scope you based on your requirement.