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

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.

When using Jenkins in the master/slave mode, setting this value to 0 would prevent the master from doing any building on its own. Slaves may not have zero executors, but may be temporarily disabled using the button on the slave's status page.