deśa-bandhaḥ-cittasya dhāraṇā
Concentration is fixing your mind in one place/direction
Concentration (dhāraṇā) can be defined in many ways:
You can deliberately choose the direction or object of concentration
Your entire attention is focused and maintanind for a certain period pf time
During this time, you put aside or rise above any distraction(s) that arise
The object of concentration may be gross or subtle, material or spiritual (examples follow in the later Sutra-s).
When you can achieve the above without failing, this level of concentration is the preliminary condition for reaching the next stage.
Damian Cadman-Jones is an authorised teacher of Prana Vashya Yoga™