WebAssembly is a specification of a small assembly language. This language can call into functions in its host environment. The "web" part is that all evergreen web browsers have implemented this language, and you can use it from inside your browser, and call JavaScript functions into it.
This means that WebAssembly is actually broader than just the web; for example the Etherium folks have been discussing using wasm as their language to script their blockchain.
It doesn't currently open up any real new API hooks; it's mostly about being an efficient language for computation. You get near-native performance in the browser. In the future, it may or may not grow more hooks directly into the web platform, rather than needing to call into JS to do so.
This means that WebAssembly is actually broader than just the web; for example the Etherium folks have been discussing using wasm as their language to script their blockchain.
It doesn't currently open up any real new API hooks; it's mostly about being an efficient language for computation. You get near-native performance in the browser. In the future, it may or may not grow more hooks directly into the web platform, rather than needing to call into JS to do so.