It's more than the back, chaps. Top line is the line of the whole length of the horse from between the tip of its ears through to the root of its tail.
Top line is built by correct diet and working the horse in the correct way. Its a way a horse looks more than anything, a good muscular neck and back without being over or undermuscled. Its really hard to explain. Food alone won't create topline, it is about the horse being worked correctly from behind and worked in a correct outline, between hand and leg which builds the correct muscularture. Sorry not explaining very well.
If you go to my post here and scroll right down to Kallibear's explanation (2nd from bottom post), I found it really helpful - I think I 'get it now'. Before her explanation I had no idea what constituted 'topline'.