computer help

i am doing a computer upgrading and maintaince course and i am trying to figure out what it means to show a table with a 3 input or function, as on this table it has a,b and c with 2 o under a and same under b and under c it has a o and a 1 ,now under d there is nothing. i read almost all my txt book and it says nothing  ???so i am hoping some1 out there can help me.
   any help would b greatly appreciated
        mrs marilyn dixon