Your Apple ID is the key to many important things you do with Apple, such as purchasing from the iTunes and App Stores, keeping personal information up-to-date across your devices with iCloud, and locating, locking, or wiping your devices. Two-step verification is a feature you can use to keep your Apple ID as secure as possible.
Using Apple's free Find My iPhone app for iOS or a SMS/text message you can have a secondary level of authentication in order to access your Apple ID.
Visit support.apple.com for how to set it up. Unfortunately for my fellow Canadians, it's not being offered in Canada at this time:
Initially, two-step verification is being offered in the U.S., UK, Australia, Ireland, and New Zealand.