Chapter 6 Dhyana Yoga Verse 15

Shloka
Yato yato nishcharati manashchanchalamasthiram;
Tatastato niyamyaitad aatmanyeva vasham nayet.
Translation
From whatever cause the restless, unsteady mind wanders away,
from that let him restrain it and bring it under the control of the Self alone.
Interpretation
This sloka is in continuation of the previous sloka where Lord Krishna talks about a steady mind and how it meets the self. In this sloka he focuses his attention on the state of unsteady mind or a restless mind. He says that mind usually wanders here and there because of the anxiety and the physical energy exhibited by that person. Then the next step to be followed for him is to first restrain himself from those causes. Physically restraining the mind from those causes will automatically bring the mind under the control of the self.
Extending Interpretation to mankind’s Life!
Why do you get restless? You get restless because of two reasons. One, when your targets are not met and second when your physical energy doesnt match with your mental speed. If we observe the first reason, we set the targets based on what we want or what others want or what kind of recognition and power achieving that would bring to you. You never look at the task dispassionately and keep a target for himself. Suppose you want to get a job. How many of you really accept your limitations and abilities and keep a deadline for securing the job? Hardly anyone. Everyone keeps a target like by next month I need to get a job and if I start applying from today, I will surely get a job by next month. And that never happens because you have never considered your capabilities and the things you need to get sorted before setting a deadline.
Second is why our physical energies dont match our mental speeds? Body and mind are one, but exist as two different entities. One has to integrate mind with body to achieve desired result. A Chris Gayle scoring a century or a Shane Watson scoring a century at the age of 35 is because they were able to synergise the power of mind with the power of body, which creates a super power to achieve many things. Physical fitness drops faster than metal fitness and if one is able to integrate body and mind he can remain fit in whatever he does for a longer time compared to others. All this is also a form of meditation and talks about bringing synergy between the body and mind and controlling the mind from wandering away.