A Torx screw has a six-pointed, star-shaped head, as opposed to the more common cross-shaped or circular shape of other screws.
The points of the star are evenly spaced around the circumference of the head, and the shape of the head allows a Torx driver, which has a matching six-pointed tip, to be inserted into the screw head.
This allows the screw to be tightened or loosened using the Torx driver, rather than a standard screwdriver.
The size of the screw is indicated by a number, such as T10, T15, or T25, which corresponds to the width of the screw head in millimeters.