Help me in understanding a flashing LED circuit
I'll get you started. With any circuit analysis it helps to start from a known state, so let's assume  that you've just turned the circuit on and C1 is fully discharged so we can consider it to be a short circuit. Let's also assume that the battery is giving out 6 volts.

Then R1 and R2 form a potential divider, and the voltage across R1 is

6*R1/(R1+R2) = 132uV

So with a Vbe as low as that, Q1 is turned off, no current flows through R3 and Q2 is also turned off; the LED is not lit.

The current flowing through R1 and R2 now starts charging C1. After a certain length of time, the voltage across C1 and R1 becomes high enough to turn on Q1 and current begins to flow through R3.

Let's see if you can take it from there.