Hybrid EFI systems retain the familiar BIOS environment with the ability to utilise EFI capabilities.
EFI specification has been around since pre 2003 which was the renaming of a project started in 1998. EFI provides advantages over using traditional BIOS systems.
Manufacturers wanted to enable their hardware to utilise these features and different manufacturers used different methods in achieving this. Hybrid EFI systems retain the familiar BIOS environment with the ability to utilise EFI capabilities. As Hybrid EFI systems do not use an EFI shell prompt or EFI GUI which are not part of the UEFI specification means there may be additional steps in order to fully enable this feature and make managing this type of system easier regardless of operating system is installed.
EFI is the predecessor of UEFI which was first collaboratively developed in 2005.
For more information regarding the definition and difference of EFI and UEFI see UEFI about and UEFI specs
Sources: Wikipedia's UEFI page and rodsbooks
