How to reduce spacing between chapters in table of content?

You can use the tocloft package and its \cftbeforechapskip command to achieve your desired spacing:

\documentclass[12pt,a4paper,fleqn]{report}
\usepackage[titles]{tocloft}
\setlength{\cftbeforechapskip}{3pt}
\begin{document}

\tableofcontents 

\chapter{introduction}
\section{first section}
\section{second section}

\chapter{Main matter}
\chapter{Results}
\chapter{Conclusion}
\chapter{Appendix A}
\chapter{Appendix B}

\end{document}

OUTPUT:

Or you could use package tocbasic:

\documentclass[12pt,a4paper,fleqn]{report}

\usepackage{tocbasic}
\DeclareTOCStyleEntry[
  beforeskip=.2em plus 1pt,% default is 1em plus 1pt
  pagenumberformat=\textbf
]{tocline}{chapter}

\begin{document}
\tableofcontents 
\chapter{introduction}
\section{first section}
\section{second section}
\chapter{Main matter}
\chapter{Results}
\chapter{Conclusion}
\chapter{Appendix A}
\chapter{Appendix B}
\end{document}

Result: