At face value it appears to be a 'double not' operation on the variable current_user. But that didn't make much sense to me at the time. I wondered if it was some special Ruby operator, but I could find no reference to it.
After a couple of experiments I see now that it is indeed a double not operator. It serves here as a very concise way to return true or false depending on whether the variable is nil or not. Here is how it works in an irb shell:
>> foo = nil
>> foo = 1
Concise is good, but not at the cost of being unclear. I'm on the fence about this one.