Out of the box WASM programs cannot interact with anything outside of the WebAssembly sandbox. It can only call functions that the host (whatever is running your WASM) has declared.. WASI is a standard set of functions that you can call from your WASM program so that you can interact with the host machine's file system, network and other things. It is modelled after POSIX. It provides some standardization for WASM programs for doing these types of things and guarantees some basic level of support. Otherwise, each host WASM program could implement them differently and WASM programs wouldn't really be portable.