What is a "Overfull \hbox (9.89561pt too wide)"?

The allowed space between two words is given by the interwordspace (\fontdimen2\font) and interwordstretch (\fontdimen3\font). If it is not possible to set a paragraph with these lengths (with hyphenation) TeX will complain with a overfull or underfull box. Setting the \emergencystrech to a value greater than 0pt it can stretch the interwordspace to a bigger value. You can set this value individually or using \sloppy or the environment sloppypar

\documentclass[draft]{article}
\usepackage[textwidth=4.5cm]{geometry}
\usepackage{blindtext}
\parindent=0pt
\begin{document}
interwordspace: \the\fontdimen2\font \\

interwordstretch: \the\fontdimen3\font \\

emergencystretch: \the\emergencystretch\par
\blindtext

\newpage
\begin{sloppypar}
interwordspace: \the\fontdimen2\font \\

interwordstretch: \the\fontdimen3\font \\

emergencystretch: \the\emergencystretch\par
\blindtext
\end{sloppypar}

\end{document}