Dharana (concentration) is the sixth limb of Patanjali's eightfold path. Its purpose is to help the mind get to a point that it's ready for meditation once the senses have been withdrawn during Pratyahara.
This is usually put this into practice by finding something for the 'monkey mind' to focus on (also known as a Drishti) such as a candle, your navel, your breath etc. Binding your consciousness with something physical allows your mind to steady and prepare for deep meditation. This is also a great way to reduce anxiety and slow down in our chaos-filled lives!