What is a WINEPREFIX?

WINEPREFIX is an envirnomental variable.

It sets where Wine looks for its configuration and files, in esence where it puts its C: drive.

By default it is $HOME/.wine.

Some programs require very specific configurations and some people will use a specific prefix for different programs.

See the Wine wiki for Wineprefixes for details on how to do so and various options.

A Wineprefix is like a small windows system that is run through wine. A clean Wineprefix is a brand new one that does not have any programs installed in it or settings changed. What you can do is have a separate Wineprefix for every program you install on wine to make removal easier and to prevent conflict. I think that is what Winetricks does. The default one is stored in your home folder in .wine.