When somebody is behind you on a bike and they'd like to pass, they ring their bell. This sounds very normal, but we don't actually do it in the US. Sometimes kids get horns on their bikes, but more often than not there is nothing, and the rider just yells "passing" or something.
But in the morning, when you haven't quite woken up, bells are very nice. They're not mean, they aren't saying "you're in my way", they're just saying "hallo, be careful"! I think that they're much more friendly. And they're pretty too. It's nice to hear bells in the morning.