Chain guides are devices which keep the chain tensioned preventing chain slap or the chain coming off the chainring when landing jumps or going over very rough terrain. They are usually in the form of a small cage above or below the chainset, some have both cages, through which the chain travels through helping keep it in a straight line coming off the chainring. A jockey wheel in the cages keeps the chain running smoothly under tension reducing the amount of slack in the chain between front and rear rings.
Chain guides used to be devices only downhillers and dirt jumpers used but with the advent of single ring cross country chainsets, double chainring compatibility of new chain guides and bigger travel rigs, they are becoming a lot more common.
Chain guides come in several fitments:
ICSG fits directly on to special tabs on the frame (frames are made with this attachment and cannot be retro fitted)
BB Mount which fit on to any frame round the bottom bracket area between frame shell and cranks - you may need to run a slightly wider BB or use less spacers to accommodate this.