Dharana is the sixth limb of ashtanga yoga and first limb of Raja Yoga.
Reducing the thoughts means controlling the thoughts.
The mind should be arrested or stopped and brought back to the object of concentration.
While practising dharana, turn the gaze inward to the Spiritual Heart where the God resides as the Self or the Atman.
Dharana helps the individual to take up one idea and that idea becomes his main concern.
Dharana leads to know and investigate the latent powers of the individual being.
The object you select to practice dharana, in the beginning, serves you as an object to fix your mind or to control your mind.
This is a story from the epic Sriman Mahabharatha.
Every object, however small or big or huge it is, has a significance, a nature and a quality.