Yoga sutra iii.11 “sarvarthata ekagratayoh ksaya udayau cittasya samadhiparinamah”

“The weakening of scattered attention and the rise of one-pointed attention in the citta is the transformation toward samadhi.”
Our minds are like a busy intersection and our thoughts are the hundreds of vehicles passing through it.  When there is a lot of traffic how are we able to reach a destination?  Well the easiest way is to choose a route with less traffic.
Learning how to reduce our thoughts makes it easier to concentrate and pay attention to one thought.  When we learn to slow the movement of the mind we are able to concentrate longer, to pay attention when someone is talking, to really listen.  When this restlessness is weakened and we are able to focus our attention we can move towards one pointed attention of the consciousness.
The ability to focus our attention on an object and increase the duration of this concentration will lead us to absorption which is samadhi.  When we are able to focus out attention until we loose ourselves, loose sense of time and place this is the type of absorption experienced in samadhi.
When we are able to create this state by choice we have “weakened the scattered attention, transforming the citta, consciousness, towards samadhi”.  It is the act of conscious restraint that allows the transition from many thoughts ,to one thought, to no thought.  This is the state of samadhi, complete absorption, not on one thought or point but on nothingness.  To be absorbed by a thought until it disappears but the concentration stays absorbed by the emptiness.