The topline of a horse is basically its back - from the poll to the dock. Horsemen will critique a horse's topline or top line as part of discussing overall conformation.
However, a "poor topline" is more often an indication that a horse is unfit and out of shape, especially in riding horses. Being ridden tends to build muscle around and above the spine. Therefore, any horseman will look at a horse's topline to see how fit that animal is and how much it has worked lately - which can also indicate how it might behave when you put a saddle on it. (Trust me, riding a horse that hasn't worked in a while is not always fun).
This is a fit horse with a very good topline. Note the slight elevation of the hindquarters above the spine - in this case a sign of draft breeding.