Yesterday I had a chance to listen to Carrie Gray speak. Carrie is a UX, UI, IA guru. During part of her talk she focused on MediaQueries and how useful they are when designing for todays consumer. Everyday millions of sites are visited by consumers using their smartphone, tablet, netbook, etc. Screens keep getting smaller, and when designing that needs to be kept in mind.
Technically speaking, A media query consists of a media type and zero or more expressions that check for the conditions of particular media features.
In laymen terms, Media Queries will simplify your life when designing with the mobile device in mind. Media Queries intelligently resize and rearrange your content depending on the size of the users screen. You can hide certain things, rearrange others. Super useful!