The simplified answer is that WebAssembly allows applications written in high-level languages, like C#, to be downloaded and then run in your browser.
WebAssembly is also known as WASM, which will give it a better explanation of what it is. WASM stands for Web Assembly State Machine, which is basically a small stack-based virtual machine, also known as a sandbox, that runs in a browser, or theoretically, any imbedded device you want to.
So how does WebAssembly work? Let say you have a piece of code you wrote in C# or any other high-level language. Your compiler can then compile your code to the instruction set of the WASM virtual machine, usually, this gets saved as a binary format in a .wasm file. So basically it converts your C# to a binary format that the WASM Virtual machine can understand.
Because the WASM virtual machine is designed to compile to real processors, this binary WASM file can now be used by the runtime, in our case the browser, to compile and execute our converted C# code.
Company’s like AutoCAD was able to convert its CAD drawing program with WASM to work in the browser, that is the power of WebAssembly.