Countersunk bolts are bolts designed to fit inside a countersunk hole. A countersink is both the recessed hole and the tool to make the hole that allows compatible screws and bolts to screw in flush to a surface. Countersunk bolts are mostly hidden when screwed in, because their heads are shaped to fit within the special holes. When these holes are empty, they look like a normal screw hole with a conical opening at the top; when they are full, the only visible part should be the very top of the screw or bolt.
When viewed in a cross-section, a countersink looks a lot like a standard screw opening. The hole is cut and threaded the exact same way as a standard screw hole. The only real difference is at the very top of the opening, where a specialized piece of machinery cuts a conical opening that looks a lot like a funnel in cross-section. This conical opening allows countersunk bolts to screw into the hole and down into the opening, so the bolt head is flush with the surface of the object.
There are two main reasons to use countersunk bolts. The first is to create a smooth finish on an object without bolt heads sticking out. Some manufacturers take this a step further and use the countersink to completely cover the bolt. The countersink is slightly larger than it needs to be and the bolt ends up recessed in the opening; its head is below the surface. A material is placed on the bolt head and is covered, likely for the life of the object.
The second reason to use countersunk bolts is to prevent the exposure of sharp edges. When a hole is cut into a material, the edges are often very sharp, especially if it is cut into metal. The sharper the angle, the sharper the edge is likely to be. A traditional bolt hole is cut using a right angle, but the conical hole used by countersunk bolts uses a much wider angle, reducing the overall sharpness of the cut. By the same token, this also removes burs and color variations made when the bolt hole is initially cut.
In addition to providing the above benefits, countersinking is used in other ways. One of the more common additional uses is in a process called dimpling. The conical holes are placed in sheet metal in long rows and columns. This increases the structural stability of the metal and allows the sheets to stack on top of one another.