This controls the number of concurrent builds that Jenkins can perform on this agent. So the value affects the overall system load Jenkins may incur. A good value to start with would be the number of processors.

Increasing this value beyond that would cause each build to take longer, but it could increase the overall throughput, because it allows CPU to build one project while another build is waiting for I/O.

Setting this value to 0 is useful to remove a disabled agent from Jenkins temporarily without losing other configuration information.