ForageWeb.ForageView.forage_horizontal_error_tag

You're seeing just the function forage_horizontal_error_tag, go back to ForageWeb.ForageView module for more information.
Link to this function

forage_horizontal_error_tag(form, field, error_helpers, class)

Renders validation errors inside a horizontal form group.

By default, you should not have to use this function. The functions defined when you use ForageWeb.ForageView, ... in your view module already take care of rendering errors the right way.