A short circuit operator. 

x OR y - if x is false, then y, else x (1). 

(1) This is a short-circuit operator, so it only evaluates the second argument if the first one is false.

The first argument is 'if'
The second argument is 'then'
The third argument is 'else'

Move to the second argument if the first is false. 
Evaluate the first argument 'if x is false' - look at the second argument if the first argument is false - 'then y'.