Current limit and current gain bjt question
What is the voltage across R2? 

When the voltage across R2 reaches Vbe-on, what happens with Q2? 

Answer those questions and come back with more questions of your own if necessary. 

As a side note, it is customary to place a low-value resistor (1k) in series with the base of Q2. This prevents damaging the base of Q2 when a sudden short occurs at the output.