Smart Compose helps you write your emails faster and more efficiently by suggesting the rest of your sentence for you while you're typing it. If you tend to use the same structures and phrases frequently, it can save you a ton of time. The feature is already live in Gmail on the web and was promised to be coming to Android, starting with the Pixels. The short-term exclusivity seems to be more restricted though, as Smart Compose is showing up on the Pixel 3 but not on the Pixel 2 / 2 XL yet.
Artem found that the feature is working on his Pixel 3. The suggestions are greyed out and he can simply swipe to approve them. There's a setting option for Smart Compose too, where you can turn the feature off if you prefer to avoid it.
None of these options appear to be live on our Pixel 2 XL devices, so it seems like the option is being triggered remotely only for the Pixel 3. Those of us with older Pixels will need to wait.