Here we have 555 adjustable timer with two alternate blinking LEDs. Pin 3 of 555 IC can source or sink the current. When output of pin 3 is enabled, it will source LED 1 and it will light up. When the output is turned off, it is sinking the current at that pin and power will route through 9V vcc into pin 3 and it will turn on the LED 2. Duration of the on/off timing of each LED will depend on value of resistors and capacitor. If the timing cycle (duty cycle is adjusted for 50 / 50 %, LEDs will be on and off for equal time as of each other. Otherwise, one LED will remain on longer than the other one. At a time, only one LED will be on. Read more about duty cycle.
