A large LaTeX project may be best split into multiple files, each containing a well-defined section of the bigger work. This split makes it easier to manage the project.

The root file in a multi-file project is the one containing a document block of the following type,

\begin{document}
...
\end{document}

Note this refers to LaTeX. A plain TeX root file will have a different, simpler markers in it (for instance a \bye command).

Subfiles are included into the root file using \input{...} or \include{...} commands.

\documentclass[11pt]{article}
\begin{document}
abc
\input{a.tex}
abc
\include{b.tex}
\end{document}

where files a.tex and b.tex will be placed in the same directory and contain text without a document block,

Contents of a.tex

Contents of b.tex

Together the three files root.tex, a.tex and b.tex will form a multi-file LaTeX project. Any large piece of work such as a thesis or a book may be better served by a multi-file project structure.