Hold your horses means wait, slow down, or be patient. People usually say it when someone is rushing ahead, getting too excited, or trying to act before everything is ready.
The expression comes from a very physical image. If you were handling horses, you sometimes had to hold them back so they would not bolt, lunge forward, or start moving before the right moment. Because that action was easy to picture, English speakers began using it figuratively for people too.
Today, it is usually informal and slightly playful. It can sound friendly, but tone matters. Said sharply, it may sound impatient.
- Hold your horses, we have not heard the full plan yet.
- Hold your horses, let me finish explaining the rules.
A close meaning is wait a second or not so fast. The difference is that hold your horses often suggests that someone is eager or charging ahead, which matches the original horse image.