What is BIOS and UEFI?

BIOS (and its modern successor UEFI) is the firmware that starts your PC before Windows loads. Its setup screen controls boot order, hardware toggles, virtualisation and security features like Secure Boot.

Enter it by pressing a key at power-on (commonly Del, F2 or F10), or from Windows: Settings → System → Recovery → Advanced startup, then Troubleshoot → UEFI Firmware Settings. Change only what you understand — wrong settings can stop the PC booting.