Clamping arms with screws (red parts)
These clamp onto the wheel rims to hold the device securely.
2. Black rod / bar
Provides structural rigidity and holds the measuring components.
3. Mirror / reflective plate
Used for reflecting the light beam from the opposite gauge to measure the alignment angle.
4. Graduated scale / protractor dial
Allows you to read the toe-in or toe-out angle (in degrees or millimeters).
5. Adjustable sight or laser (on some models)
In this case, it looks like an optical/mechanical sighting system rather than a laser.