Gutterman, chain guards on rotary mowers (bush hogs) are a series of short chains hanging vertically down in front of the openings on the back and front of the mower. Rotary mowers can throw a rock or other hard object a considerable distance and be a danger if anyone is in the area. There are stories of rocks from a mower breaking windows, penetrating the siding of a barn or doing serious damage to an individual and the chain guards help prevent this. Even with chain guards it is best to make sure the area is clear of people when mowing. Hope that helps explain the term.
MarkV