In short: Google debuted Smart Compose for Gmail back at Google I/O in May. It was available on the desktop version of Gmail. But earlier this month at the #MadeByGoogle hardware event, the search giant announced that it was coming to the Gmail app on Android. Smart Compose is apparently rolling out now on the Pixel 3, and is not appearing on other devices just yet. It will be coming to other Android smartphones in the future, but it looks like this is going to remain a Pixel 3 "exclusive" for now, as we have tried to find the settings for Smart Compose on other devices (including a Pixel 2 XL and Galaxy Note 9) and were unable to do so.
Smart Compose is a way for users to write less, when they are doing emails. Google is reading through your emails (which it was already doing, anyways) and gets context for how you reply and compose emails. This allows it to use that context to help you write out emails and even finish sentences for you. For example, if you always use the same line in an email to someone, Gmail will know that and bring it up for you to either press tab (on desktop) or swipe (on mobile) to finish that sentence. It actually makes composing emails so much easier for many people. It's a useful feature on desktop, but on mobile, it is going to be even more useful. This is because it is going to allow you to do less typing, than usual. Which for some people, can be pretty tricky on a smaller device like a smartphone.
Background: This is part of Google's machine learning suite of features that it has been working on for a few years now. Google has been using machine learning and artificial intelligence in just about every product that it has, and this is because it believes that machine learning and artificial intelligence is the future, and it will also help users get more done in less time. Some are likely going to be upset that Google is reading their emails to get this context, but the reality is, Google has been reading your emails for years. This is how it shows you relevant ads, as well as populating your Google Now feed (back before it turned into the Google Feed and now Discover) with tracking numbers of packages being shipped, or when bills are due and so forth. So this is nothing new for Google, and no one should be too worried about it.
Impact: Smart Compose is likely going to be way more helpful on mobile than it is on desktop, and that's because more emails composed on mobile are going to be shorter and to the point. But it is good to see Smart Compose coming to Gmail on mobile finally. There's still no word on when it will be available for all Android smartphones, and not just the Pixel 3. But for now, it's only available on the Pixel 3. This is likely something that will be a limited time thing, however.