Does it mean to hide codes of jacascript?
WebAssembly (Wasm) is the 'executable' binary format for an abstract machine. The web browser implements this abstract machine allowing it to execute .wasm files. Wasm is designed to be executed at near native speed; well, nearer to native speed than Javascript can be.
Since Wasm is an executable format, programming languages can target it. That's another benefit of Wasm:
Write programs for the web browser in your favourite language.
Actually, modern computers are so fast, and desktop computers typically so idle, people have compiled interpreters (Python, Javascript, etc) into Wasm, then use the web browser's Wasm engine to run the interpreter to run Python/Javascript/whatever scripts. Layers upon layers of compilation/transformation and still fast enough to be useful and/or fun.