What is it?
Yap is a visual programming environment, much like Max/MSP and Pure Data. Like Pure Data, yap is an open source product.
Yap patches can be handled natively with the YSE sound engine, without even using yap. YSE runs on multiple platforms (Windows, Mac, Android and iOS) which makes yap patches very portable.
Should I use it?
Yap is very new. It does not have the maturity of Pure Data or Max. Its main advantage right now is for developers: the interface is available as a nuGet package for C# and can be integrated in your own projects.