@Jeykey Let’s split it on parts:

user_path(@conn, :create)

This one generates local path which type is String.t(). Local path is relative part to your server which means that it does not include protocol, domain and port information for example: /users/create. This path would be used in HTML form element in action attribute.

user_path/2 is function imported from your router Helpers sub-module (for example MyAppWeb.Router.Helpers). This one is generated based on your routes. For more details please see documentation pages.

Name assigns usually is a map which is used by Phoenix templates.

Map.put(assigns, :action, user_path(@conn, :create))

Map.put/3 function is really simple. For any map it adds or overrides (if key exist) specific value which could be accessed by specific key. For example:

original_map = %{a: 5}
modified_map = Map.put(original_map, :b, 10)
modified_map.b # returns: 10

Finally render/2 function simply renders specific template (here: "form_new.html") with specified assigns (here: assigns with added :action key)

Helpful resources:

1. Map.put/3 information at Elixir documentation
2. Helpers information at Phoenix documentation
3. Assigns information at render/3 documentation