It means that some one has voted a 56 on an image, the only number higher than ten that can be used to vote for an image. Bestowed upon only the greatest of images, this rare vote usually boosts a c-index to above 400. This almost always guarantees it a place in the permanent galleries, unless some one else votes the incredibly rare negative 56,000.
Just kidding. It can also be interpreted as "I don't like this image, so I am choosing to give it an improper fraction".