Sharing a great example of ruby's & operator:

https://stackoverflow.com/questions/9429819/what-is-the-functionality-of-operator-in-ruby

By prepending & to a symbol you are creating a lambda function that will call method with a name of that symbol on the object you pass into this function. Taking that into account:

ar.map(&:reverse)

is roughly equivalent to:

ar.map { |element| element.reverse }