More than 90% of today's teens multitask. But it can have some very negative effects. If teens multitask too much, it can have an effect on their brain, positive or not, and change the way they learn. It can make you feel good, but you actually are getting less done in more time. It can also lower IQ points. If you are a multitasker, you need to watch the things you do.
There are different types of multitasking. You can eat while studying, and it will have almost no effect. Except that you aren't hungry anymore! Some people listen to music while doing homework, and that is OK too. Listening to music uses a different part of your brain. You can get distracted by it, but it is not really considered “multitasking.”