Earlier today, Microsoft released Windows 10 for PCs build 16237 to the Fast ring, and also announced that the final Bug Bash for the Fall Creators Update will kick off next week. But the Insider team isn't done for the day, as it's now offering PC build 16232 to the Slow ring.
This is a much bigger deal than the average monthly Slow ring build, as their hasn't actually been one since the Creators Update RTM in March. There have been cumulative updates that were exclusive to the ring, but this is the first preview of the Fall Creators Update.
If you're on the Slow ring, you're going to experience a ton of new features. OneDrive Files On-Demand is the Windows 10 replacement for Windows 8.1's placeholders. There are lots of improvements to the onscreen keyboards as well, such as a one-handed Word Flow-style keyboard, and a shortcut to an emoji picker. That's just the very tip of the iceberg though; for more, check out the release notes from builds 16215 and 16226.
Microsoft says that it's a staggered rollout, so you might not get the build right away. Typically, ISO images should follow a Slow ring release, so if you can't wait, you can always just install it when those become available.