In short, Audio Worklet is a new API which allows you to write code to generate or modify an audio signal in real time. The key point is that it runs in a separate browser thread, i.e. it doesn't have to wait for the browser painting the screen or loading stuff from the network to do the calculations necessary to generate the audio signal. This allows much smoother and continuous sound, something that was not possible before. Worklets share the interface with normal audio nodes, which means that they can be connected with other nodes (such as pre-made sample players, oscillators, filters) and even microphone / line in inputs.

This application allows you to write, test and save / share them. The idea is to have an online playground to share DSP code, like a sort of JSFiddle, but for audio.