As we all know, machines read binary codes (strings of zeros and ones) because it is very convenient for them but not for humans. So engineers basically created a mapping from binary code and called the mapped version 'Assembly'.
WebAssembly is an 'Assembly'-like language. It's nickname is wasm. Basically WebAssembly is the Assembly language for modern browsers.
People usually don't write Assembly code directly because it can be tedious. That's why there are many Assembly compilers that simply create Assembly code from C/C++, C#, Rust, Java ... So basically you can run an existing C/C++ or Rust application inside a browser.