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.
In the last post we find that the soul is covered by layers of the subtle body and gross body. Each layer covers or alludes the soul in disassociating with its true self. This is called ego or thinking that we are the body.
Arjuna's chariot is an analogy which illustrates the roles each layer of the subtle and gross body plays. The chariot carries the passenger (Arjuna) in the same way that the material body is the carrier of the soul. The chariot can only be driven with the presence of the charioteer which is the intellect. The battle ground is the object of the senses. The 5 horses are the 5 mighty senses which are tamed only by the reins (the mind). The mind is superior to the senses and even higher than the mind is the intellect or the charioteer that grips the horses reins.
The soul teamed with intellect drives the body and instructs the mind to control the senses. If the intellect is weak and unable to control the mind, then the senses will run wild like untamed horses.
The soul or Atma sits besides the super-soul or Parmatma who is an expansion of Krishna himself. The super-soul is the divinity within that acts as the witness to all our endeavours, thoughts and prayers and our eternal best friend that gifts us with intuition and guidance. Krishna in the form of super-soul is within the heart, sitting patiently until the day that the soul breaks through the layers of the gross and subtle body and realises his true self, remembering his eternal relationship with Lord Krishna.