There are two circuits in the above figure.

The first Figure is a flashing light circuit. When the circuit work, the two LEDs will also light glimmered. The principle of the circuit 555 works similar to the audio oscillator, the difference is to increase the capacity of capacitor Cl to 4.7μF. Therefore, the circuit's oscillation frequency is very low, NE555 of 3 feet high and low potential changes slowly.  When the 3-pin output high, the light-emitting diode VDl, VD2 also power light. When the  3-pin output low, the two LEDs are off. R3 in the circuit the greater the resistance, the smaller brightness; R3 resistance are smaller, the greater the brightness. It is worth noting, R3 resistance should not be too small, otherwise the current flowing through the LED is too large,  the circuit power consumption is large on the light-emitting diodes will have adverse effects, or even burned.  Typically, the current flowing through the LED can be controlled between 10 ~ 20mA better.

The second figure using 555 is a core flash circuit, their normal work, light-emitting diode VDl and VD2 will take turns alternately flashing circuit summarized as follows: time-base circuit NE555 and Rl, R2, Cl composition of low-frequency oscillator. Circuit from the oscillator, the time base circuit 555 of the 3 foot high and low potential of changing as high as 3 feet when, VDl loss of power does not shine,  VD2 power LED; as low as 3 feet when, VDl power light, VD2 loss of power not light. Therefore, the two would take turns flashing LED light.  Concrete production, the two light-emitting diodes use red, green or yellow, the difference can be more eye-catching number.