If you use your computer often enough, you may be familiar with the feeling of making accidental mistakes on the keyboard, whether it be due to deleting a batch of text you didn't mean to get rid of or whether you're just indecisive and ultimately decide to stick with something which you've previously undone. In these instances, the keyboard shortcuts for the "undo" and "redo" actions are immensely helpful as they can help you recover lost text or redo an action. Say you type up a paragraph for a thesis and then you delete it because you aren't happy with the way it sounds. Then you decide you want it back so you hit the CTRL+Z keyboard shortcut to undo your deletion, only to end up back at your former decision of wanting to delete the entire paragraph and start from scratch. So you hit CTRL+Shift+Z to redo.
These kinds of actions are going to be getting native support within Android M, so users will finally have access to the undo and redo keyboard shortcuts. The catch is that at the moment within the developer preview, these actions are only supported in native android when people are using a Bluetooth keyboard to type on their device. Within the M preview using the onscreen keyboard the shortcuts are still not supported.
Perhaps Google will add the support for the onscreen keyboards within the final release, although there is no way of knowing that for now, and since this is the developer preview there is even a chance Google may decide not to keep the keyboard shortcut support for Bluetooth keyboards once Android M enters a final version and launches out to users. If you've got the Android M developer preview running on your device and you have a Bluetooth keyboard handy, give this little pair of actions a try for yourself and see how they work. Out of all the features so far that have shown up in Android M, this is one of the most minor changes but it's still interesting to see.