Modules add extra functionality to PixTudio, as PixTudio on its own is just a language and a virtual machine. The default modules contain the basics you need to make a game: video, audio, input, etc. More advanced external libraries are available, like network and 3D rendering.
Enabling a module is easy, using import:
import "<modulename>"
Some external libraries use a header (.INC) instead, so use include:
include "<includename>"