Change is constant in the physical or manifested world of objects and the unmanifested or atma remains changeless. So, a mechanism is needed to bring coherence between these two worlds. Metaphorically, it's like the ball bearing mechanism between a stationary hub and a rotating wheel or like a gearbox handling two different speeds from the engine and wheels. Similarly, it's a mechanism of senses, mind (manas or chitt) and intellect (buddhi) between the changeless atma and the ever changing world of objects. Krishna gave a hierarchy (3.42) that the senses are superior to sense objects, the mind is superior to the senses, and superior to the mind is the intellect. Superior even to the intellect is the self or atma.
The physical part of the senses automatically responds to changes in the physical world. The mind is a combination of the controller part of the senses along with memory and is expected to handle every outside change brought to it by the physical part of the senses to keep us safe. The main issue is whether sensual stimuli control the mind or the intellect. It would be a reactionary life if dictated by stimuli and a life of awareness if guided by intellect.
That's why Krishna says to start the practice of using the intellect little by little to let the mind be established in the self (6.25) and encourages to do this practice with determination and enthusiasm (6.23). Contemporary literature also suggests that 10,000 hours of practice is needed to master any skill.
In the process, we need to abandon even sankalp (intention and planning) and restrain senses (6.24). Restraining senses is nothing but restraining the desire of getting sensual stimuli of our choice. Krishna assures that once we attain the supreme bliss of transcending senses, we will not be moved even by great sorrows (6.22).
Source - Daily World