The chariot of the body.
The five horses represent the five senses (tongue, eyes, ears, nose and skin). The reins, the driving instrument, symbolize the mind, the driver is the intelligence, and the passenger is the spirit soul.
Wonderful eye-opener from Kathopanishad:-
Atmanam rathinam vidhi
Shareeram rathameva tu
Budhim tu sarathim vidhi
A model for the working of the principle through the intellect, mind and body is described here. Know the spirit within as the sole passenger sitting in a chariot. The chariot is the body associated with that spirit. The intellect sits in the driving seat and controls the horses harnessed to the chariot. Mind is the reins by which the charioteer gets hold of the horses, to make them perform in accordance with its will.
The sense organs are the horses. The objects in this universe keep pulling the sense organs towards them. The intelligent persons understand that the inner spirit is the true consummator of all objects. The spirit acts through the mind to reach the objects through the sense organs.
Yastuavijnanavan bhavatyuktena manasa sada
Tasyendriyanyavashyani dushtashva iva saratheh
When one ignores this model it disturbs the union of mind and inner spirit. Then the sense organs work without respecting any control. They run in all directions dissipating energy like a group of foul horses in the hands of an incompetent charioteer. The charioteer has no control over the horses and eventually they topple the chariot. No meaningful progress is possible in such a scenario.
Yastu vijnanavan bhavati yuktena manasa sada
Tasyendriyani vashyani sadashva iva sarathe
For one who acknowledges this model the mind is united with the inner spirit. His sense organs remain perfectly under control like good horses in the hands of a competent charioteer.
Yastuavijnanavan bhavatyanyamanaskah sada/shuchih
Na sa tatpadamapnoti samsaram chadhigachchati
By ignoring this model the mind gets scattered and turbulent. It becomes incapable of conceiving higher realities. He is trapped in the ocean of worldly transactions. He can not attain that state where one is free from the cycles of birth and death.
Yastu vijnanavan bhavati samanaska sada shuchh
Sa tu tatpadamapnoti yasmat bhooyo na jayate
When this model is acknowledged his mind remains organized and clear. He acquires the power to reach that state where one is free from the cycle of birth and death.
Vijnana sarathir yastu manahpragrahavannarah
So/dhvanahparamapnoti tadvishnoh paramam padam
One who keeps wisdom as the charioteer and use the mind to rein the sense organs attains the strength to traverse the course of a meaningful life. He reaches the state of Vishnu, which transcends all states, the fills the universe with his perennial presence