Hi,
Its hard for me understand how in above circuit the Q2 transistor is on. Please explain.

Thanks in advance

 

What is the supply voltage?
What is the voltage of the zener diode?

Given the answers to the above, is Q1 on or off?
Given that what happens on Q2 base?

 

The zener is 12V, so Q1 is off, thus Q2 is on.

The question shoud be "what use is this circuit" ?

 

The zener is 12V, so Q1 is off, thus Q2 is on.

The question shoud be "what use is this circuit" ?

The questions for you to answer. They were for the TS. The circuit is useful as a homework question or, possibly, as a voltage monitor.