What is the "path"variable in rails integration test?

I was looking at the RailsGuides and I saw this example:

require 'test_helper'

class UserFlowsTest < ActionDispatch::IntegrationTest
  test "login and browse site" do
    # login via https
    https!
    get "/login"
    assert_response :success

    post_via_redirect "/login", username: users(:david).username, password: users(:david).password
    assert_equal '/welcome', path
    assert_equal 'Welcome david!', flash[:notice]

    https!(false)
    get "/articles/all"
    assert_response :success
    assert assigns(:articles)
  end
end

I am confused about the variable "path" in the line assert_equal '/welcome', path. What is this exactly? Is this the url that would show up in your browser after we perform the post_via_redirect action right before it?

I looked into the helpers available in the integration tests, but the document didn't say anything about this "path" variable.

Additionally, I try it at my rails app and it works fine, but when I tried to call it at one of my controller tests, it gave me and undefined local variables or method error. So does it mean that the "path" variable is only available in the integration tests? What if I want to use it in the other types of tests?