The Coconut Cannon resembles a hollowed out log with a with twig sticking out of it, this is possibly the weapon's crosshair. As the name would imply, the Coconut Cannon is capable of shooting coconuts.
The Coconut Cannon can be purchased by Donkey Kong from Funky Kong for three Banana Bunch Coins at Funky's Armory in the area Jungle Japes, the first area of Donkey Kong 64. With the Coconut Cannon, Donkey gains the ability to shoot Banana Balloons, hit Coconut Switches and snipe enemies from afar.
Further into Donkey Kong 64 special upgrades to the Coconut Cannon (as well as other weapons) can be purchased at Funky's Armory. Some of these upgrades include a larger ammo capacity, a sniper scope and enemy-seeking ammo.
Interestingly, beta-screenshots of Donkey Kong 64 showed that the Coconut Cannon was originally going to be a weapon similar to a double-barrel shotgun.