All the ingredients for a command like this are available, we just have to put them together.
We define a new command \refcite based on the default definition of \cite. But in the precode we inject some code that prints "ref." or "refs." (depending on the number of citation as stored in citetotal).
You can find the original definition of \cite as used by nature.cbx in numeric-comp.cbx (ll. 209-216 in v3.16).
\documentclass[british]{article}
\usepackage[T1]{fontenc}
\usepackage{babel}
\usepackage{csquotes}
\usepackage[backend=biber, style=nature]{biblatex}
\NewBibliographyString{ref}
\NewBibliographyString{refs}
\DefineBibliographyStrings{english}{
ref = {ref\adddot},
refs = {refs\adddot},
}
\newbibmacro{cite:intro:ref}{%
\ifnumgreater{\value{citetotal}}{1}
{\bibstring{refs}}
{\bibstring{ref}}%
\printunit{\addnbspace}}
\DeclareCiteCommand{\refcite}
{\usebibmacro{cite:init}%
\usebibmacro{prenote}%
\usebibmacro{cite:intro:ref}}
{\usebibmacro{citeindex}%
\usebibmacro{cite:comp}}
{}
{\usebibmacro{cite:dump}%
\usebibmacro{postnote}}
\newrobustcmd*{\Refcite}{\bibsentence\refcite}
\addbibresource{biblatex-examples.bib}
\begin{document}
Lorem \refcite{sigfridsson}
Lorem \refcite{nussbaum}
Lorem \refcite{nussbaum,geer}
\Refcite{sigfridsson}
Lorem \autocite{sigfridsson}
\printbibliography
\end{document}

If you just want a naked citation without any brackets, you can define something like
\DeclareCiteCommand{\barecite}
{\usebibmacro{cite:init}%
\usebibmacro{prenote}}
{\usebibmacro{citeindex}%
\usebibmacro{cite:comp}}
{}
{\usebibmacro{cite:dump}%
\usebibmacro{postnote}}
Again, this definition is based on \cite, but it omits the wrapper [\mkbibbrackets].