Another way of saying "slow down" or "stop." Usually used when someone is being hasty about something and a wiser person says "Hold your horses!" to make them stop and think. Encourages someone to have caution.
Based on the times when people used wagons or carriages, and when they needed to keep them under control they would need to literally hold the horses back from running.
"Hold your horses," said Joe, "you don't know when the movie even starts."