What does it mean when your iPhone 5s is vibrating while charging?

Today I was charging my phone notification and stuff if off and on silent but then I hear my phone vibrating and I check if I got a message or email but I didn't it was from last night and there's a bolt when I was charging my phone so what does it mean when your phone vibrates even if notifications and stuff is off is that mean my charger is broken? But then my charger looks fine and I turn my notification and email off

IPhone usually vibrates when the charger is first plugged in.

It shouldn't vibrate willy nilly. If it does, then you may need a new charger cable (I've had that happen).