^ Velvet gave quite a good explanation.
However, to learn to see your distances you need to know how to develop and maintain a balanced, rhythmic, consistent and forward canter on the horse you're jumping. The better your canter, the easier it will be to start seeing a distance (because it will be consistent, not a 12 ft. Stride then getting flat and low into a 14 ft. Stride), and the better the horse will be able to jump.