When do I need a resistor during LED flashing?
Forward voltage is a voltage necessary for LED to conduct electricity. If the voltage across your LED is 3.4V, it will draw as much current as it can, and burn out eventually. So you need to limit this current to 300 mA. If you have power source of 9V, it means you can fit 2 LED comfortably in series as they will need 6.8 V together. Remaining 9-6.8 = 2.2V will be over your current limiting resistor. Since your current is 300 mA, resistor value should be 2.2V/300mA = 7.3 Ohm!