It's a scope from Nikon, that is designed to be used with shotguns, usually with rifled barrels.
Since heavy slugs, or sabots fired from shotguns, drop pretty quick over distance, this model has "bullet drop compensation" (BDC), which has little rings, visible below the cross-hairs, to put on the target at 100, 150 and 200yds.
The slughunter also has 5" of eye releif to protect your face under heavy firing recoil.
I have the newer 1.55x5x36, which is a great scope, but frankly, with my old eye sight, wished I had purchased the more powerful 3x9x40.
http://www.nikonhunting.com/riflescopes-slughunter.html