Retirement tax questions

A retirement plan like a 401k contributions would be in box 12 on your W2.  Do you have anything in any of the box 12 lines?  Code D is for 401k  contributions.     Or is it asking you if box 13 is checked?  There are 3 little boxes in box 13.  Other than box 12 & 13 I don't know what it's asking your for.  What else is on the screen?