Latex (stylized as $\LaTeX$) is a markup language used to create scientific papers. It's the language used to produce PDF documents, like the ones you'll commonly see on arXiv.

Latex has a simple declarative syntax that looks like \command[opt1][opt2]...[optN]{arg1}{arg2}...{argN}. That is to say, most commands start with a backslash \, followed by command name + arguments given in square or curly braces. For example, \textbf{Hello} makes a bold text Hello while \textit{World} makes an italic text World. “Bf” stands for “bold font” while “it” stands for “italic”. Arguments in curly braces { ... } are mandatory, while arguments in square brackets [ ... ] are optional.

Latex documents have the extension .tex. Unlike HTML – which you can view in any browser you can't readily view latex documents until you compile them with a latex compiler. The most common compiler is pdflatex. This compiler turns .tex documents into .pdf documents. PDFs can then be viewed by a PDF viewer or in the browser.

The following is a minimal latex document.

% Compile: pdflatex main.tex
% Result:  main.pdf
 
\documentclass{article}
\begin{document}
Hello world!
\end{document}

You can now open main.pdf and see the result!