When using regular variables with string interpolation in Ruby, they must be wrapped in curly braces (e.g. "This is a #{variable}"). With instance variables (and class and global variables) you can just use the octothorp followed directly by the variable.

Here is an example of this in action:

class Person
  def initialize(name)
    @name = name
  end

  def whoami
    puts "I am #@name"
  end
end

bob = Person.new("bob")
#=> #<Person:0x007fdaf3291618 @name="bob">

bob.whoami
# I am bob

This is a handy shortcut, but may affect readability and/or result in an interpolation error at some point. Your mileage may vary.

h/t Josh Davey