“The restraint of the modifications of the mind-stuff [or consciousness] is Yoga.” 
“Restraint of the modifications of mind-stuff” might sound rather hard to understand.
Orit Sen Gupta, the Hebrew translator, chose to express it as: “Yoga is the quieting of mental activity.”
It means, at least in part: “Yoga” is not doing or allowing anything to upset our own peace of mind.
Most of us get “upset.” We get upset at rudeness; at “bad luck” — large or small — that comes upon us, especially unexpectedly; at not getting our own way; and so on.
Our own upset can even become intolerable to us, especially when we don’t know how to stop it.
We’ll never stop the upset as long as we ascribe it to some external cause of which our reaction is an inevitable, unavoidable effect.
In a culture — perhaps a world — that extols money, fame, and power, it’s not surprising that the majority are unaware that they’re disturbing their own innate peacefulness. It surprises me, though, when even someone who is otherwise involved in some aspect of personal and spiritual growth — as a participant and/or as a professional — doesn’t recognize that each upset is a chance to grow in peacefulness.
Swami Satchidananda says of this Sutra of Patanjali’s:
“In this Sutra Patanjali gives the goal of yoga. For a keen student this one Sutra would be enough because the rest of them only explain this one.” 
Personal peace of mind integrates all aspects of spiritual practice — diet, meditation, physical practices, Scriptural study, and so on. It’s at least part of why we do them.
But in the end, true peace is brought about by making changes in our thinking.
Of course, there are levels of mental peace. We could say, there are levels of mental quietness. At each moment of true peace, however brief, it’s almost impossible not to respond with some feeling of gratitude.
I’ve worked for my peace. If nothing else, I’ve learned that it requires continual monitoring.
I can share peace, sometimes. But I can’t change a person’s thinking for them.
I hope this is a year of quiet minds for all of us.