class Liquid::Increment

increment is used in a place where one needs to insert a counter

  into a template, and needs the counter to survive across
  multiple instantiations of the template.
  (To achieve the survival, the application must keep the context)

  if the variable does not exist, it is created with value 0.

Hello: {% increment variable %}

gives you:

Hello: 0
Hello: 1
Hello: 2