What does the phrase "Underfull \hbox (badness 10000) in paragraph" actually mean?

Question

What does the actual phrase above mean?

Answer 1

99 times out of 100 it means you have \\ incorrectly placed at the end of a paragraph. But to dissect the message:

Underfull \hbox (badness 10000) in paragraph at lines 4--5

A box is underfull if there is not enough content to fill its stated size.

If you go \hbox to 5cm{A} then it makes a horizontal box (\hbox) 5cm wide just containing an A so it is underfull and will generate a warning, the exact amount of badness depends how much any white space is over-stretched, but here there is no white space so it is infinitely bad, which is arbitrarily truncated to the maximum value, 10000.

By placing \\ at the end of a paragraph you force a line break but there is nothing at all in the forced final line of the paragraph so it is a box that is \textwidth wide with no content. It appears a bit like vertical space but it is not it is a spurious line at the end of the paragraph. So for example it does not stretch and is not dropped at the start of a page.

Answer 2

I just find out that using new line command \\ in latex (overleaf) causes the problem.

If you end a line with \\ and start with a nonempty line then it works fine.

Example:

\begin{document}
Hello this is the first line.\\
This is the second line.\\ 
\end{document}

But when I need to skip one line (like pressing enter twice) I must use \\ \\ (i.e. \\ twice). Since we are putting \\ at the end of an empty line this causes the problem.

Example:

\begin{document} 
Hello this is the first line.\\
\\ %This is an empty line
This is the third line.
\end{document}

Solution: Use a text in white color between \\ and \\ to resolve this, though xcolor package will be required for this. Here is the code.

\begin{article}
\usepackage{xcolor} 
Hello this is the first line.\\
{\color{white}-}\\ % This line will now show in pdf and no error will be generated.
This is the third line.
\end{article}

Hope this works!