Formation training teaches smooth predictable flying. The formation leader must act as navigator and look out for the other aircraft in the formation and be instinctively alert to their needs. The remaining aircraft must stay on station in close proximity to the leader following his every move and command with confidence.
There is no formal "formation rating" to be earned, but for obvious reasons it would be rash to attempt to fly in close formation without training first, and indeed most flying organisations insist on it.
Before your first formation flight you will be briefed in the principles of formation flying, formation flight planning and briefings, the standard reference points for station keeping, formation changes, and hand and radio signals.
You will then start practising flight exercises paired with another pilot, each of you accompanied by an instructor. Beginning with stream take-off and landings you will initially learn station keeping in straight and level flight, turns, climbs and descents. From this you will progress to changing formation positions: echelon left to right, right to left echelon to line astern and back. As you grow in confidence and ability you will experience formation take-off and landing and finally "tail chasing" the leader.