GRUB Legacy is any version of the Grub bootloader that is older than GRUB 1.9x / Grub 2, such as 0.97.
GRUB Legacy is the first generation of Grub, with version numbers of the form 0.9x (e.g. Grub 0.97). Grub Legacy (then called Grub) is the default bootloader in Ubuntu 8.04 hardy. Since Ubuntu 9.10, Grub 2 grub2 (with version numbers 1.96 and above) is the default bootloader on new installations, but Grub Legacy may remain on upgraded installations.