Decompiling assets
A shipped game contains only compiled files.
Source2 Viewer opens the VPKs, shows what is inside a compiled resource, and decompiles it back toward its source form.
Some information is dropped at compile time, so a decompiled file is a reconstruction and not the original. Also
Source2 Viewer does not necessarily support all file types fully.
In the viewer, any supported file has Decompile & Export on its right click menu, and so does a whole folder. The CLI does the same thing without the browsing.
Further reading
The
Source2 Viewer documentation covers the tool itself in more detail: