What does Legacy Free Support in the BIOS mean?
Typically "Legacy Support" means providing a translation for DOS or bootable utilities to use USB keyboards and mice, so those programs think they are working with the older PS/2 keyboards and mice there were designed to work with. Sometimes the legacy option will provide a similar mapping for USB floppy drives, hard drives, and such.
Usually you can leave it off unless you are trying to boot into something and your keyboard or mouse isn't working. It's quite pointless for Windows XP or later.
It would help if you gave more context to the phrase.
One answer could be the removal of support for technologies considered these days to be "legacy": no PS/2 ports, no serial or parallel ports, no floppy-drive controller, no ISA bus, etc.
Another answer could be that the BIOS is based on the UEFI spec, and not the proprietary BIOS implementations that were reverse-engineered from the original IBM PC.