The big change in this build is Timeline - this is a brand new major feature that's designed to allow you to more easily see what you were doing at some point in the past. i.e. a timeline on your PC activity. As Microsoft put it:
Timeline is here! We know that it can be difficult to get back to stuff you were working on in the past. If you’re like us, you might sometimes forget which site or app you were using or where you saved a file. Now you can finally close apps without worrying—with Timeline, you can get right back to where you left off.
Timeline introduces a new way to resume past activities you started on this PC, other Windows PCs, and iOS/Android devices. Timeline enhances Task View, allowing you to switch between currently running apps and past activities.
The default view of Timeline shows snapshots of the most relevant activities from earlier in the day or a specific past date. A new annotated scrollbar makes it easy to get back to past activities.
There’s also a way to see all the activities that happened in a single day—just select See all next to the date header. Your activities will be organized into groups by hour to help you find tasks you know you worked on that morning, or whenever.
And if you can’t find the activity you’re looking for in the default view, search for it.
In Timeline, a user activity is the combination of a specific app and a specific piece of content you were working on at a specific time. Each activity links right back to a webpage, document, article, playlist, or task, saving you time when you want to resume that activity later.
App developers are working hard to enhance their apps by creating high-quality activity cards to appear in Timeline. In this Preview release, you can see and resume web-browsing activities in Microsoft Edge, files you opened in apps like Microsoft Office including Word, Excel, PowerPoint, and OneNote, and updated UWP versions of Maps, News, Money, Sports, and Weather.
In addition to this, there's the usual raft of fixes and improvements to Edge, with Cortana getting a bit of love as well. The full release notes are here