Inter-App Audio was first introduced in iOS 7.
The idea behind this technology is pretty similar to the concept of VST/AU plugins for Mac/PC. This technology allows to stream music between apps.
Inter-App Audio allows audio apps that are instruments, effects, or generators to publish an output object which can be used by other audio apps. An app that publishes an output is called a node. An app that connects and uses one or more nodes is called a host.