Some Android phones come with their own notification light - it's a great way of knowing you have a notification on your device without having to turn it on to check.
The only problem is usually you don't have much control over that LED and what it can do. Light Flow is an app that changes that.
Light Flow allows you to have a lot more control over your LED and notifications. You can assign a number of different colours to basically every app that provides some sort of notification. You can also assign colours to contacts so you know if a notification is important or not.
The LED can also flash in succession to show you have multiple notifications. You can even set notifications at different priorities so the most important flash first.