We all love live tiles; they're a powerful notification tool that aren't a drain on your battery
and can quickly and easily display relevant information from an app. But what if an app needs to notify you of something and you don't have it on your start screen?
You can only have so many tiles before the screen feels bloated, and though the new size options
should help fit more on the screen without it being a chore, if you're a heavy app user and prefer a sparse start screen, you've got to rely on toast notifications; they're less than perfect and if you miss them, they can't be easily viewed again like a live tile can. Microsoft has thus far avoided the traditional notifications center other mobile operating systems have implemented, but it seems they've decided to tackle this issue by using the lock screen in a decidedly minimal fashion.
The list of icons presumably stand for missed calls, texts, and email, or perhaps some more general system notification section. According to WPCentral, this is only the first piece of the new system and there will be something more developers can use in this new lock screen, though they decline to comment further and simply dangle the tantalizing tidbit for us with the promise we'll see more in weeks.