A post hole digger is a tool used to dig narrow holes to install posts, such as for fences and signs.
There are different kinds of post hole diggers. The pictured type is jabbed into the ground in the open position until the blades are buried. At that point the handles are pulled apart to close the tool and grab the chunk of soil loosened. They are then pulled out of the ground with the chunk of soil. The process is repeated until the hole is deep enough, or until the hole is so deep and narrow that the handles can no longer be pulled apart fully. This is one of the weaknesses of this kind of post hole digger.
There are also auger type post hole diggers. These are generally easier to use, and they can be used to a much greater depth, as the hole can be dug as deep as the entire length of the shaft. More importantly, they form a much neater hole, with a well-defined circumference. However, the very fact that the person doing the digging is guaranteed a perfect hole with an auger post hole digger is also its greatest weakness: any given auger style post hole digger can only dig one size hole—a hole with the same diameter as the auger's screw.