What happen when you freeze object? Or variable?

obj_var_freeze.rb

class U
  def initialize
     @uu='cdefaflejlfkj'
   end
   attr_accessor :uu
 end

 u=U.new.freeze

u.uu
# => "cdefaflejlfkj" # first letter is small "c"
u.uu=2 # we cannot do this, ok
# RuntimeError: can't modify frozen U
# from (pry):93:in `__pry__'

u.uu.capitalize!
# => "Cdefaflejlfkj"
u.uu
=> "Cdefaflejlfkj" # capitalized!


u.uu.freeze
# "Cdefaflejlfkj"

u.uu.reverse!
# RuntimeError: can't modify frozen String
# from (pry):105:in `reverse!'

u.uu
#=> "Cdefaflejlfkj" # still the same