Ruby and "instance variable not initialized"

instance variable not initialized.rb

irb(main):001:0> baz
Traceback (most recent call last):
        4: from /Users/dentarg/.rubies/ruby-2.7.1/bin/irb:23:in `<main>'
        3: from /Users/dentarg/.rubies/ruby-2.7.1/bin/irb:23:in `load'
        2: from /Users/dentarg/.rubies/ruby-2.7.1/lib/ruby/gems/2.7.0/gems/irb-1.2.3/exe/irb:11:in `<top (required)>'
        1: from (irb):1
NameError (undefined local variable or method `baz' for main:Object)
irb(main):002:0> baz = "baz" if false
=> nil
irb(main):003:0> baz
=> nil
irb(main):004:0> $VERBOSE = true
irb(main):005:0>
irb(main):006:0> @baz = "baz" if false
=> nil
irb(main):007:0> @baz
(irb):7: warning: instance variable @baz not initialized
=> nil
irb(main):008:0> @baz = nil
irb(main):009:0> @baz
=> nil