Commentators in horse trotting and pacing events describe a horse as racing without cover. What does this phrase mean?

Answer: In horse racing some horses need "cover" because they don't produce their best efforts when leading from the front. so the jockey keeps the horse behind a couple of other runners, which provide this "cover", until the final stages of the race when the horse is encouraged to do his best... I guess this applies also to trotting races.