On the web new APIs often have vendor prefixes while they're in an experimental state. For example the AudioContext API is known as webkitAudioContext in Safari at the time of this writing. The vendor_prefix attribute indicates these alternative names, which are used if the normal name isn't defined.

For example to use AudioContext you might do:

# #![allow(unused_variables)]
#fn main() {
#[wasm_bindgen]
extern "C" {
    #[wasm_bindgen(vendor_prefix = webkit)]
    type AudioContext;

    // methods on `AudioContext` ...
}
#}

Whenever AudioContext is used it'll use AudioContext if the global namespace defines it or alternatively it'll fall back to webkitAudioContext.

Note that vendor_prefix cannot be used with module = "..." or js_namespace = ..., so it's basically limited to web-platform APIs today.