Help interpreting a simple jQuery-related instruction

Help interpreting a simple jQuery-related instruction

Hi, I'm new to jQuery and relatively new to Ruby on Rails. I've successfully installed the rails3-jquery-autocomplete gem ( http://github.com/crowdint/rails3-jquery-autocomplete) and I'm playing around with it in a test Rails project. 

One of the instructions on the gem states:
===============
Getting the object id

If you need to use the id of the selected object, you can use the :id_element HTML tag too:

f.autocomplete_field :brand_name, autocomplete_brand_name_products_path, :id_element => '#some_element'

This will update the field with id *#some_element with the id of the selected object. The value for this option can be any jQuery selector.
===============

That seems simple enough. My question related to jQuery is -- what does the last sentence mean? Can I just have a <div id="some_element"> on the page? Or do I need to write some jQuery code in order to make it a jQuery selector?

Thanks for the help with this simple question.