One look at an email can rob you of 15 minutes of focus. One call on your cell phone, one tweet, one instant message can destroy your schedule, forcing you to move meetings, or blow off really important things, like love, and friendship.

This quote is saying that if you take a look at an email or receive a call, tweet, or instant message, it can take away 15 minutes of your focus and concentration. Moreover, it can significantly disrupt your schedule and make it difficult to adhere to it or leave you no room for important activities such as spending quality time with family and friends.