An AudioWorkletProcessor (often abbreviated as AWP in web synth code and docs) is a tool from [web-audio] that allows running custom use-defined code directly on the audio thread. It exposes a message-based interface for exchanging arbitrary data with the main thread as well as supporting other methods such as SharedArrayBuffer.
WebAssembly can also be compiled + instantiated from within the AWP, meaning that DSP code written in Rust can be compiled and run in WebAudio via AWPs. This is the method that most of the low-level DSP code in web synth is implemented including the [granular-synthesizer], [fm-synth], [sample-editor], and various other components and [module]s.