Microsoft is rolling out a small configuration update to the Windows 10 Mobile Insider preview for those on the Fast ring. According to Windows Insider chief Gabriel Aul, Insiders shouldn't expect to see any new features or changes in this update, as it's solely meant to prepare devices to receive future cumulative updates. From Aul:
"Today we are rolling out a Configuration Update to the Windows 10 Mobile Insider devices in the Fast ring. This update prepares devices to receive the next cumulative updates, as well as improvements to the client-side update experience. There are no noticeable changes immediately after installing this Configuration Update."
In any case, if you're on the Fast ring, you should either be alerted to the update or be able to manually pull it down to your device relatively soon.
Source: Gabriel Aul (Twitter)