Gaelic is the pre-English language spoken traditionally in northern and western Scotland (the Highlands). It's closely related to (and most think derived from) Irish, still the official language of Ireland.
Gaelic was spoken in much of the rest of Scotland as well, but replaced in those areas by Scots (simplistically an English/Gaelic mix, but a full language in its own right) and later English as the everyday spoken tongue starting as much as 800 years ago.
Only about 50,000 native Gaelic speakers (1% of the population) remain in Scotland, but the language, after nearly dying out, is experiencing an upsurge of interest by Scots and others who are studying it.
Gaelic is a beautiful language in the way it is spoken and written, but is fairly tough to learn partially because the spelling doesn't correspond well to the way words are pronounced.