Imagine you've just finished composing a detailed and helpful email, and it's time to add a subject. How can you effectively sum up this masterpiece of electronic communication? So... "Hey" it is. Subjects can be hard, but Gmail will start filling those in for you with Smart Compose soon.
Smart Compose arrived on Gmail as an official feature last summer, and it later came to Pixel phones as an exclusive. Just last month, it expanded to all phones. Now, the next frontier for Smart Compose is not the body but the subject line. As with body suggestions, you can autofill the Google subject line by pressing Tab. Google doesn't mention phones, but presumably the feature will show up there eventually.
Starting today, G Suite accounts on the rapid release timeline will start getting subject line suggestions based on the text of your email (15 days for full availability). Scheduled release domains won't get the update until April 23rd with the same 15-day window.