A regular instance method can be used by all the instances of that class and all of them will inherit their behavior from its class. But what if we want a particular instance to have a specific behavior that no other instance can have that is where singleton method is being used.Singleton methods are the methods that can only be accessible to a single object only.
In some language this can be done by defining another class, which can only be instantiated once, but in ruby we use singleton method for that.

For example:

class Welcome
  def greet_user
    puts "Hello all"
  end
end

user1 = Welcome.new
user1.greet_user
user2 = Welcome.new
user2.greet_user
   
p user1
p user2

This will print:

Hello all
Hello all

Now we want instance user1 to have a specific behavior, for that we define a method that can be accessible to user1 only

For example:

class Welcome

end

user1 = Welcome.new
user2 = Welcome.new

def user1.greet_user

  puts "Hello admin"

end

p user1.greet_user
p user1.respond_to?(:greet_user)
p user2.respond_to?(:greet_user)
p user2.greet_user

The output will be:

Hello admin
true
false
undefined method `greet_user' for #<Welcome:0x00000002f67348>

Here respond_to? method is used to check whether the object can respond to the given method or not