The endpoint stack is the full set of layers that make a PC work: the operating system, device drivers, applications, and vendor software that controls hardware features. On a Windows system, a problem may not come from the app a user sees, but from a lower layer such as a graphics driver, audio package, or security agent.
This matters in cyber security because attacks and defenses both depend on these layers. Malware often targets drivers, services, or bundled device software to gain persistence, hide activity, or weaken security tools. Defenders also have to manage the stack carefully: patching the OS without checking driver compatibility can break critical functions, while updating endpoint protection or firmware can close attack paths. In practice, the endpoint stack is where routine maintenance, hardening, and stability all intersect.