It builds your C++ code with G++ or VC++'s CL.
BYP is still in heavy alpha. If there's a bug, add it to the issues list and hopefully I'll get to fixing it. If you decide to fix a bug yourself, just pull request it and I'll merge it in!
If you want to start using BYP, I'd suggest reading the Getting Started guide.
To install BYP, run the following command. Node.js 0.6.x and npm must be installed for this to work properly.
npm install byp -g