The following illustration shows how near-far problem affects communication.
https://www.tutorialspoint.com/cdma/images/near_far_problem_affects.jpg
As shown in the illustration, user A is far away from the receiver and user B is close to the receiver, there will be big difference between desired signal power and interfered signal power. Desired signal power will be much higher than the interfered signal power and hence SN ratio of user A will be smaller and communication quality of user A will be severely degraded.
