The Yoga Sutras are a collection of verses (or 'threads': each the barest thread of meaning). They are attributed to Patanjali, although this may not be one single person, since the estimated date of the sutras ranges from 5000 BC to 300 AD.
The yoga sutras are arranged in four Books:
The restraint of the modifications of the mind-stuff is Yoga.
The world outside us is based entirely on our own mental attitude to it, so, if you feel constrained, then you are constrained, and if you feel free, then you are free. If you can control your mental attitude and change it as you want, then you have achieved the goal of yoga and experienced yoga.
Then the Seer (Self) abides in His own nature.
You are not your body or your mind. You are the Knower or Seer that can see the body acting and the mind thinking, without being involved with them. To achieve this state, the mind must be quietened so that it does not distort the truth. When the mind is still, it becomes as clear as a still lake and you can see your true Self.
There are five kinds of mental modifications which are either painful or painless.
These mental modifications are restrained by practice and non-attachment.
Of these two, effort towards steadiness of mind is practice. Practice becomes firmly grounded when well attended to for a long time, without break and in all earnestness. ( A story is told here of the two men asking how many more births they had to have before they could go to heaven. The one who was patient and willing to do whatever was asked in an earnest and settled way was immediately taken to heaven, whereas the other who was impatient, unsettled, anxious to get the work done was left to wait.)
The flow of nirodha parinama becomes steady with habit. When there is a decline in distractedness and appearance of one-pointedness, then comes Samadhi parinamah.Then again, when the subsiding past and rising present images are identical, there is ekagrata parinama (one-pointedness).