Samsung has had a rich screenshot editor in TouchWiz for years, and Apple even added a similar feature to iOS 11. Until now, stock Android has lagged behind in this area, but that's finally fixed in Android P.
Screenshot editing in Android P is actually rather intuitive. To start, just take a screenshot like you normally would. For most phones (including Pixel and Nexus devices), that's as simple as holding the volume down and power buttons simultaneously for a second or two.
Once you've taken your screenshot, you'll see a new button on the heads up-style notification — it says "Edit." Just tap this "Edit" button and you'll immediately be taken to a menu where you can edit the screenshot.
It's just calling on the editing activity from Google Photos, though, so it's pretty bare-bones compared to Apple and Samsung's offerings. You can't draw on the screenshots, add arrows, or do any other form of highlighting, though that could easily be added in a future update to Google Photos.
For now, this at least gives us stock Android users a simple way to crop screenshots, which is a big step in the right direction. We'll keep digging into the Android P developer preview that Google just launched to see if we can find any other interesting features like this, so stay tuned for more coverage!