It's the 9th division of the jyotish chart. It typically relates to a person's romantic relationships in his or her life. But you can also see your relationships with others in life, such as your mother, father, brothers, sisters, cousins and friends. You can even see your relationships with coworkers and bosses.
The navamsha chart gives a little bit more details and nuances of the relationships. By knowing these you can understand a deeper insight of your love life. Specifically, one should look more deeply on planets that are neecha bhanga in the navamsha. By definition these planets are weak, but they have countervailing effects that you can perceive that are positive. For example, the 12th lord, signifying losses or expenses in the 6th house and in neecha bhanga can mean a hidden source of income. Or that your cousins or workers somehow gain from your relationship with them.