If this is the question bouncing the database is nothing but shutting down the database and bring the database online.You can bounce a database by using Server manager that is
svrmgrl
Command from Unix prompt or Dos prompt.
This is performed depending on the requirment
1) You might need to bounce if you change one or some of the parameters in the parameter file
2) Or you might be taking a cold backup of the database.
3) Restarting your system for hardware maintaience or after making changes to your operating system.