A bucked shin is a swelling on the front of the cannon bone between the knee and fetlock. It's an inflammation of the sheath that covers the bone and is most often seen in racehorses that are starting their career and other two-year-olds being worked hard. It's caused by concussion and can be treated by pin firing (which requires two or three months of rest and which some vets consider not helpful) or by backing off on training and giving the horse lighter intensity work until the swelling goes down. (Not complete rest).
Bucked shins are essentially caused by working young horses too hard, which is why they're often seen in the racing industry where there's a lot of pressure to get two year olds ready for their first race.