The GBU-39 Small Diameter Bomb is a special type of bomb that fits in the palm of your hand and is designed to be very precise when it goes boom.
It's kind of like a really, really small airplane that you can fill up with lots and lots of tiny bombs. When it's dropped from a bigger airplane, it can glide down to where it needs to go using special fins on its body.
Once it's close to its target, it can explode with a really big boom, but only on the exact spot it needs to hit, like a bad guy's hideout or an enemy tank.
Think of it like throwing a small rock at a target, but with the accuracy of a superhero. Except the rock is packed with explosives and the superhero is a plane.