It is the shorthand for a logical OR operation. It is equivalent to:
a || a = b
Note: The above code sample has been corrected to reflect the true (if unintuitive) behavior if expanding a ||= b. Thanks to the people who pointed that out for me. Here is the source
if a evaluates to true it will remain as is, otherwise b will be assigned to a. In ruby nil evaluates to false, so you can see how this is useful for lazy loading and default value assignment.
