You can set up a PIN via the Passcode Lock feature on the Settings screen (in the More tab). Your Passcode is a four-digit code that allows you to secure your account access upon closing the app (or navigating to another app on your device).
Note that this added security measure is different from your account password. After a period of time, you will still be prompted for your account password. You can select this period of time from the Settings screen as well (every time, 5 minutes, 15 minutes, or 30 minutes).