Dharana translates as concentration. By clearing the mind of outside distractions, one is better off to achieve Dharana and let go of inner distractions. The goal is to be able to focus solely on one single thing at a given moment and keep the mind completely fixed on that one thing. When we concentrate long enough, we allow ourselves into a meditative state of mind.
Some teachers will invite students to create an intention for their asana class, having this single focus throughout can be a practice of Dharana.
Intentions are usually those of a single word or short phrase (a mantra) repeated mentally to focus the mind on a positive thought or something the practitioner is in need of in the present moment.
Examples include: Gratitude, Self-love, Courage, Strenght, Happiness, Forgiveness, Positivity, Presence, Contentment, Loyalty, Confidence, etc.
Sometimes focusing solely on the poses or the breath can also work as Dharana.
Dharana is about mastering one's awareness.