This is the complete specification of webassembly (wasm) text and binary format. It contains some formalism is meant more for language designers, implementation developers, than it is for users of wasm code.
And for those who still don't have a clue what webassembly, basically, it is a safe, portable, low-level code format designed for efficient execution and compact representation. Its main goal is to enable high performance applications on the Web, but it does not make any Web-specific assumptions or provide Web-specific features, so can be employed in other environments as well.
If you are the curious type, you should dig through the pages and become a wasm ninja.
Read the full spec at: https://webassembly.github.io/spec