This tunable enables/disables NMI watchdog. It is enabled by default on configurations that support it.
On many x86/x86-64 type hardware there is a feature that enables us to generate 'watchdog NMI interrupts' (NMI: Non Maskable Interrupt which get executed even if the system is otherwise locked up hard). This can be used to debug hard kernel lockups. By executing periodic NMI interrupts, the kernel can monitor whether any CPU has locked up, and print out debugging messages if so.