What is a rootkit and why is it so difficult to detect?

What will be an ideal response?

A rootkit is a virus that loads itself before the OS boot is complete. It can hide in boot managers, boot loader programs, or kernel mode device drivers. UEFI secure boot is especially designed to catch rootkits that launch during the boot. Because it is already loaded when most anti-malware software loads, it is sometimes overlooked by the software. A rootkit can hide folders that contain software it has installed, cause Task Manager to display a different name for its process, hide registry keys, and can operate in user mode or kernel mode. This last trick helps it remain undetected.