The full docs are https://www.cesanta.com/developer/smartjs (linked from the github repo).
TL;DR: two pieces: multiplatform firmware focusing very small embedded devices + hosted services that help you build your IoT solution.
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The Smart.js firmware implements a bunch of networking protocols and a JavaScript interpreter that fits in <1KB of ram.
> Is it a web server/protocol ? Is it some other kind of control layer We do provide a JSON+WebSocket based protocol and some tooling around it that help you build applications. It's designed to play nicely with devices that are not always online (e.g. power saving) and that might often change network.
However Smart.js it's not only that. You can use plain HTTP and plain WebSockets and (soon) MQTT and connect your device with whatever is the rest of your stack.
> What data does it deal with, etc. We also offer a hosted service that can collect metrics, store logs, do pubsub messaging and perform unattended OTA updates of your firmware.