A managed bean (sometimes simply referrred to as an MBean) is a Java object that matches the JavaBean specification. The bean is automatically created, and has a lifetime that depends on the scope it has.
What are the differenct scopes?
How to use them in JSF?
Have a look into http://www.java2s.com/Tutorials/Java/JSF/0060__JSF_Managed_Beans.htm for more details of use manged bean scope in JSF.