A lot of companies on the Web are providing support for secondary authentication. Some do this with apps like Google Authenticator, while others just use SMS. Essentially, both methods provide you with a security code that you need to enter in order to log in.
LinkedIn has joined the two-step verification trend, using SMS as the delivery method for codes. Here's how you set it up:
Step 1: Log in to your LinkedIn profile. Click your user picture/avatar in the upper-right-hand corner and select Settings.
Step 2: On the tabs along the bottom-left-hand side, click Account. Then, click Manage security settings (bottom of the left column).
Step 3: Under Two-step verification for sign-in, click Turn On. Enter a reliable cell phone number and then click Send Code.
Now you just need to enter the code you receive via SMS and log back in to the LinkedIn Web site.
If you need to change the phone number associated with your account, or remove two-step verification, those changes can be made easily from the same menu.