What's the purpose of assigns method in Rails tests (MiniTest)?

It means if a controller defined an instance variable @item="something".

You can fetch an instance variable in your test with e.g.:

# It will check if the instance variable is a string.
assert_kind_of String, assigns(:item)

Be aware assigns deprecated in Rails 5. And extracted to separate gem. To use it you must include 'rails-controller-testing' to your gemfile.