The pseudo-method nil? determines whether an expression's runtime is Nil. For example:
a = 1
a.nil? # => false
b = nil
b.nil? # => true
It is a pseudo-method because the compiler knows about it and it can affect type information, as explained in if var.nil?(...).
It has the same effect as writing is_a?(Nil) but it's shorter and easier to read and write.