documentclass{article}

usepackage[bookmarks=true]{hyperref}

begin{document}

tableofcontents

section{Background}

Here is some Background.

section{System}

This is the system under study.

The variables are:

section{Results}

subsection{Result 1}

We got some results.

subsection{Result 2}

And then some more results.

end{document}

BOOKMARK [1][-]{section.1}{Background}{}% 1
BOOKMARK [1][-]{section.2}{System}{}% 2
BOOKMARK [1][-]{section.3}{Results}{}% 3
BOOKMARK [2][-]{subsection.3.1}{Result 1}{section.3}% 4
BOOKMARK [2][-]{subsection.3.2}{Result 2}{section.3}% 5