Meaning:

Wait; Be patient and careful; Wait and listen to what someone's saying.

You say "Hold your horses" to someone when he/she is impatient
or wants to do something immediately without thinking about his/her action and consequences carefully.
You tell him/her to be patient and wait.

Example:

Hold your horses. We are not ready to go yet.

Hold your horses. You might find a better one for the same price in another store.

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