Once you have signed in to online banking, you have begun an "active session." If you walk away during the course of an active session, someone else can use your computer to perform transactions involving your accounts. It is similar to walking away from an ATM while your card is still in the machine.
For your security, online banking will automatically end your active session if there is no activity on your session for 10 minutes. However, to eliminate any exposure, it is best to end the active session by clicking the "Log Off" button.