Android O will include the picture-in-picture feature which means that users will be able to keep videos open as they play on top of of another screen. This wouldn't be the same as the multi-window feature which allows users to keep two apps open simultaneously, but instead is likely to be more akin to the way YouTube videos play in a small player at the bottom right corner of the screen after you minimize them which allows you to continue scrolling through YouTube and looking at other videos. The change here would be that users will have the ability to do the same type of thing but while in other apps as well.
According to the details users will be able to adjust the aspect ratios of the apps they're using in the picture-in-picture mode so they can make the sizes of a window as big or as little as they want, this way they won't be limited and it can more easily allow them to enjoy a video while searching for something in Maps or browsing through apps on the Play Store.
In addition to giving users the ability to change the sizes of windows for open apps in this mode, Google says there will be interactive buttons for things like videos that will allow users to pause and play videos so they don't have to exit out of picture-in-picture. Google is also adding in multi-display support, though they were vague on specifics and merely stated that this would allow uses to launch an activity on a separate display, like a monitor for example. Picture-in-picture was first rumored to be part of Android O at the end of last week as a handful of the features included in this version of the software were leaked out. That rumor also mentioned the adaptive icons which is indeed another element that is part of the first developer preview. Google has released the Android O Developer Preview 1 for developers to get their hands on so they can check it out and prepare their apps for compatibility, and although it's only intended for developers and not daily consumer use, consumers can still get their hands on the software and flash it as well.