I'm going to add a filter to a page index, with the options of [both, true, false].
This is basic HTML stuff, but if you have a form input like a dropdown (select), the value you choose in that element is only sent to the server if it is inside a form element, and you submit that form in some way to the server. On the server, you need to permit that attribute in strong parameters, so it won't be filtered out, and then use that in your controller method to act as your filter.
If you're on an index page, say www.example.com/pages, then you can code the form like this:
<%= form.for url: '/pages', method: :get do |f| %>
<%= f.collection_select :filter, %w[both true false], :to_s, :to_s, onchange: "this.form.trigger('submit')", prompt: true %>
<%- end -%>
That's going to submit to your controller in the index method, where I presume you will do the rest.