We are introducing a new optional feature that adds another layer of security to your LinkedIn sign-in: two-step verification. Most internet accounts that become compromised are illegitimately accessed from a new or unknown computer (or device). Two-step verification helps address this problem by requiring you to type a numeric code when logging in from an unrecognized device for the first time. This code will be sent to your phone via SMS. When enabled, two-step verification makes it more difficult for unauthorized users to access your account, requiring them to have both your password and access to your mobile phone.
Learn more: http://blog.linkedin.com/2013/05/31/protecting-your-linkedin-account-with-two-step-verification