What is short circuit evaluation??

It means in a conditional statement with two conditions the second condition is evaluated only when the first condition is not enough to determine the value of expression

Consider a conditional statement echo "Hello" if a && b. In && condition if the first condition is false (considering a is defined as false) then the result of a && b is always false. So in this ruby won't try to evaluate b.

In the above image when a is false it neither print "Hello" nor raise any error. Means b is not evaluated.

But when the a is true, a is not enough to determine the condition so the ruby will try to evaluate b. But since b is not defined the ruby will raise error saying undefined local variable or method b