Best bet is to look at the README file in that modules folder: https://github.com/espruino/BangleApps/blob/master/modules/README.md
It covers pretty much exactly what you need to know about developing with it :)
@Gordon started
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
Best bet is to look at the README file in that modules folder: https://github.com/espruino/BangleApps/blob/master/modules/README.md
It covers pretty much exactly what you need to know about developing with it :)