If multiple people send you an iMessage, instead of all of those messages clouding your notifications tray, they'll be pulled together into a single notification letting you know you have multiple messages.
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This will in theory work with any of your apps on the
with iOS 12, so when you swipe down to see what you missed, you'll be able to focus on the highlights more quickly. You can also clear all of an app's notifications with a single swipe.
Android has offered similarly grouped notifications for a while, so this will be a nice quality of life upgrade for iPhone users. We'll look forward to testing out if it really does make managing notifications easier.