Safe mode puts your Oak into a state where you can access it again just like you did when doing the initial setup, and also stops any buggy code from locking it up again.
To access safe mode:
Disconnect the power from the oak
Put a jumper/link/wire between P1 and GND
Power up the Oak
After 2-3 seconds, remove the jumper link between P1 and GND
The Oak should now be doing a triple blink pattern like the one above, indicating it is now in safe mode.