**Metaprogramming** refers to the idea of writing and editing the code at runtime. The basic idea is that classes can be defined and redefined during run time. 

It is used throughout Ruby to implement multiple functionalities. 

# create class decision
class Decision
# create method agree
  def agree
    puts "Say Yes!!"
  end  
end
# calling 
puts Decision.new.agree

The code above shows a user-defined class in Ruby. It creates a new class `Decision` since the class was not previously defined. During run-time, we can append the following code:

class Decision
#create method disagree
  def disagree
    puts "NOOOO!"
  end
end

Since the class had previously been defined at run-time, the compiler adds the method to the class​ and no new class is created. Ruby opens the prefined `Decision` class and appends the method `disagree` in the same class. 

# display agree
Decision.new.agree
=> Say Yes!!
# display disagree
Decision.new.disagree
=> NOOOO!

Both the methods can be called at run-time using similar syntax since they refer to methods of the same class, `Decision`.

What is metaprogramming in Ruby?

Modifying code at runtime, metaprogramming in Ruby allows dynamic class definitions and method additions.