I am pretty new to latex and pgfplots. I am trying to make a gif. Can anyone please explain what is the function of <\fr> in pgfonlayer section of the following code?
\documentclass[hyperref={pdfpagelabels=false}]{beamer}
\usepackage{lmodern}
\usepackage{tikz}
\usetikzlibrary{arrows,shapes}
\tikzstyle{selected edge} = [draw,line width=5pt,-,red!50]
\usetheme{Frankfurt}
\setbeamertemplate{navigation symbols}{}%
\tikzset{
error_00/.style={color=blue,thin},
}
\begin{document}
\pgfdeclarelayer{background}
\pgfsetlayers{background,main}
\begin{frame}
\begin{figure}
\begin{tikzpicture}[scale=1]
\foreach \pos/\name in {{(-12.117,45.857)/a}, {(-12.117,45.857)/b}, {(-7.809,45.857)/c}}
\node (\name) at \pos {};
\draw[x=1 cm,y=1 cm,error_00] (-10.628,45.857) -- (-7.809,45.857);
\begin{pgfonlayer}{background}
\foreach \source / \dest / \fr in {a/b/1/, b/c/2/}
\path<\fr>[selected edge] (\source.center) edge (\dest.center);
\end{pgfonlayer}
\end{tikzpicture}
\end{figure}
\end{frame}
\end{document}
I compile the file from this link: https://martin-thoma.com/how-to-visualize-graph-algorithms-with-latex . Additionally, I do not want to make the red line blink. I just want it to appear. Any ideas to achieve it with minimal amount of change. Thanks in advance.

\source / \dest / \fr / \posare variables within the loop. They will be in{a/b/1/, b/c/2/}. So\frwill loop as1and2and so make sense to use it in<>overlay specification. – Sigur Jul 01 '17 at 15:29<\fr->. – TeXnician Jul 01 '17 at 15:51\fris the third and last variable, you do not want1/and2/but just1and2. Otherwise, you get1/-which isn't valid. Also, this has nothing to do with any GIF, as far as I can tell. And\tikzstyleis deprecated and ought not be used. – cfr Jul 01 '17 at 23:45