Multitasking is doing two or more things at the same time. For example, watching television, reading a book or playing a video game, while listening to a lecture. A LOT of people swear that when they do this they are able to concentrate better, learn more, and are more productive. However, studies have shown that when you attempt to multitask, both activities suffer. That is, neither activity gets 100% of your attention.
For more on this check out the short video below: