SyncTeX is a utility written by Jérôme Laurens which enables synchronization between your source document and the PDF output. If your editor/viewer supports it, then you can click in your source and jump to the equivalent place in the PDF or click in the PDF and it will jump to the appropriate place in your source document.
In TeXShop, for example, Command-click does the navigation. Other editor/viewer pairs may implement the exact commands slightly differently.
The files that it creates store all of the synchronization data that make this magic possible. Gzipped versions of these files are created if you pass the
--synctex=1 option to e.g. the
pdflatex command (and other engines similarly); non-zipped versions can be created with
--synctex=-1, although for a large document these files can be quite large, so the zipped ones are generally to be preferred. See
Usually this is part of the default setting for the compilation command within most TeX-aware editors.