Biblatex `language`, `hyphenation` and `langid` - what is the difference?

Question

Some time ago I got a wonderful snippet of code from this site to help work with multilingual environments. It lets have just one field, language, instead of two, language and hyphenation. I keep having to use two fields for working with Cyrillic for some reason. Can somebody explain why fieldsource is language and fieldset is langid? I cannot figure this out and thus cannot modify the code.

\DeclareSourcemap{
    \maps{
        \map{
            \step[fieldsource=language, fieldset=langid, origfieldval, final]
            \step[fieldset=language, null]
        }
    }
}

MWE

\documentclass[12pt,a4paper,twoside]{memoir}
\usepackage[T1,T2A]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[greek.ancient,latin,english,german,french,dutch,italian,spanish,russian]{babel}
\usepackage[backend=biber,citestyle=verbose-trad1,language=auto,autolang=other,autocite=footnote,citetracker=true,ibidtracker=false,opcittracker=false,citereset=chapter,citepages=omit]{biblatex}
\ExecuteBibliographyOptions{sorting=nty,notetype=foot+end}
\usepackage{csquotes}
\usepackage{tocvsec2}
\usepackage{calc}
\usepackage{textcomp}
\usepackage{microtype}
\usepackage{filecontents}
\usepackage{hyphenat}

\usepackage{filecontents}


\DeclareSourcemap{
    \maps{
        \map{
            \step[fieldsource=language, fieldset=langid, origfieldval, final]
            \step[fieldset=language, null]
        }
    }
}

\begin{filecontents}{bib.bib}


@Book{G:2,
author = {Успенский, Ф.},
title = {История},
editor = {Успенский, Ф.},
language = {russian},
}

\end{filecontents}

\addbibresource{bib.bib}

\begin{document}

Test.\autocites{G:2}

\end{document}