The zipper is a wedge, which is a simple machine that works by putting two inclined planes side by side so that they create a sharp edge. A zipper has two lower wedges that permit it to close and a single upper wedge that allows it to open.
Wedges are used to separate things, either by cutting them or pushing them apart. They also function to hold things in place. Other examples of wedges are an ax, a knife, a fork and a door stop.
The inclined plane decreases the force needed to to raise an object to a certain height. This decrease in force makes it easier to perform work.