Previously, the Linux kernel had by default a periodic interrupt that made the kernel scheduler to balance and schedule threads running among the CPU's, this interrupt is known as the timer tick and it was generated at a fix rate (usually in the order of hundreds/thousands per second), namely referred as HZ.
The kernel serviced this interrupt regardless of the power state of the CPU.
Tickless kernel, or NO_HZ, is a configuration option in the kernel that allows it to run without a periodic timer tick. Running without a periodic timer tick allows the CPU not to be disturbed when idle and can save power because it just wakes up the CPU when needed to service the timer.
The configuration option is CONFIG_NO_HZ and is set by Tickless System (Dynamic Ticks), on the Kernel Features configuration menu.