What are patron-client systems?
I assume that you are asking about the social/political system known as the patron-client system. In such a system, social and political systems are bound together by very personal ties between clients and their patrons. The patrons ae supposed to act almost like parents to their clients. In return for the clients' loyalty and political/social support, the patrons are supposed to provide for the material needs of the clients.
This sort of system can, for example, be seen in places like Mexico. In Mexico, there have historically been elites in each area who dominate the politics and society of their regions. These people rely on their clients for political support, for labor, and at times for military support to keep them in power. In return, they are supposed to help their clients get jobs, education for their children, and other such benefits.
In other words, a patron-client system is one in which the leaders and the followers have a personal connection rather than a more legalistic, impersonal connection as is typical in places like the US.
In a patron-client system, individuals in higher positions of authority, commonly known as patrons, often provide concessions for their clients, who are in turn obligated to provide the former with certain benefits. Such a relationship is usually based on the personal ties between the two groups. Bureaucratic impartiality is often viewed to be less important than personal considerations and the misuse of public power for personal ends is generally accepted in society.