What does the black rectangle at the end of the line mean? It is overflow?
This writes overflow
\documentclass[12pt,a4paper]{report}
\setlength\textwidth{145mm}
\setlength\textheight{247mm}
\setlength\oddsidemargin{15mm}
\setlength\evensidemargin{15mm}
\setlength\topmargin{0mm}
\setlength\headsep{0mm}
\setlength\headheight{0mm}
\let\openright=\clearpage
\begin{document}
\begin{description}
\item [Epocha (ekvinokcium)]
$T$ udává okamžik v čase. Je několik druhů epoch - referenční epocha (v případě světelných křivek udává, kdy je fáze 0), epocha maximální radiální rychlosti, epocha periastra, epocha primárního minima, apod. Udává se v různých časových jednotkách:
\begin{itemize}
\item Juliánská data se hodí na analýzu dlouhých časových pozorování. Označují střední sluneční dny, které začínají v poledne světového času SČ, což je lokální čas na poledníku procházejícím observatoří v Greenwichi. Počátek těchto časových jednotek JD0 připadá na střední poledne v Greenwichi 1. ledna 4713 př. n. l.
\end{itemize}
\end{description}
\end{document}

draftmode (or with other 'debug'-like settings) those boxes indicate overfull boxes, which means in that case that the line protrudes into the margin because LaTeX can't find a good point to break it. – moewe Dec 08 '18 at 15:31\overfullrule=1mmand nothing happend. When I add\documentclass[12pt,a4paper,draft]{report}pictures and links don't work. – Elisabeth Dec 08 '18 at 16:48draftoption definitely affects the way thegraphicxandhyperrefpackages do their job. (Do you maybe not know what the word "draft" means in a document context?) Incidentally, the expression "nothing happened" is not helpful. Are you maybe saying that the document didn't compile at all? If that is not what you're saying, do try to provide a more meaningful description of (a) what you're trying to achieve and (b) what you're actually doing. – Mico Dec 08 '18 at 16:56draftsetting anywhere, either something setsdraftimplicitly or the box was enabled with a different setting. We can only help you properly if you show us a short example document that reproduces the behaviour you are seeing (https://tex.meta.stackexchange.com/q/228/35864) – moewe Dec 08 '18 at 19:43tabularxmaterial is wider than the textblock. Instead, it is one word, in one cell, that's too wide for that cell. I already informed the OP in a comment below an earlier posting how to create a fix: simply change\setlength\tabcolsep{3pt}to\setlength\tabcolsep{2pt}. I guess my suggestion wasn't taken seriously. – Mico Dec 08 '18 at 20:46babelwith the correct language option. If you use pdfLaTeX you should also load\usepackage[T1]{fontenc}to allow line-breaking of words with accented characters. – moewe Dec 09 '18 at 06:18\usepackage[czech]{babel}and I had before\usepackage[T1]{fontenc}. There is still overflow. What an abbreviation OP means? – Elisabeth Dec 09 '18 at 07:11fontencandbabel(though it is far better than before):microtypemight help, sometimes it is also recommended to just rewrite/reformulate the offending sentence so it breaks nicer (cf. https://tex.stackexchange.com/q/20585/35864, https://tex.stackexchange.com/q/209229/35864) – moewe Dec 09 '18 at 07:19\hfuzz=99pthelped – Elisabeth Dec 09 '18 at 07:35\hfuzzonly affects the warnings and not the actual typesetting. That means that the line still protrudes into the margin and will look unsightly, but you have told TeX to not warn you about it. That seems to be the worst workaround. – moewe Dec 09 '18 at 08:01