M10 is a designation for a metric screw with a diameter of 10 millimeters.
The letter "M" stands for "metric" and indicates that the screw has a metric thread pitch. The number "10" indicates the diameter of the screw in millimeters.
Metric screws are measured in millimeters, and the thread pitch is the distance between threads on the screw. The thread pitch of an M10 screw is typically 1.25 millimeters, although this can vary depending on the manufacturer and the specific type of screw.
Metric screws are commonly used in Europe and other parts of the world, but they are not as widely used in the United States, where fractional-inch screws are more common. In the United States, an M10 screw would be equivalent to a screw with a diameter of about 3/8".