what's -@ and +@ in Ruby

what's -@ and +@ in Ruby

what are +@ and -@ in Ruby?
If you put a string stick with + and -, you will see NoMethodError
pry> +""
NoMethodError: undefined method `+@' for "\n":String
pry> -""
NoMethodError: undefined method `-@' for "\n":String
I found them are in something called TkVariable What is their job? What's
typical scenario to use them?
Thanks a lot!!!