It's the third parameter to the call you're making, a boolean. False is
not checked, true is checked.

http://api.rubyonrails.com/classes/ActionView/Helpers/FormTagHelper.html

i.e., in your above example,
 <%= check_box "category", category.id, category.my_boolean_attribute?
%>
Would check the checkbox if category.my_boolean_attribute? returned
true and would leave it unchecked otherwise.