This distributor uses a light beam produced by an LED (which is like a small light bulb), to shine light on a photosensor (an electronic device which responds to either light, or the absence of light). The LED and photosensor are called "optics' because they use visible light to do their thing. There is a rotating shutter wheel with alternating open slots and solid spaces, which is located between the LED and the photosensor. The solid spaces on the shutter wheel are referred to as "towers." The shutter wheel is a round plastic wheel, maybe an inch in height and a few inches in diameter, with 8 narrow slots in it (if it were used for an 8 cylinder engine; or 6 slots for a 6 cylinder engine, etc.).
When an open slot in the shutter wheel lines up between the LED and the photosensor, the light shines through the slot to the photosensor, and the sensor signals the module to turn off the charging current through the coil, which then produces a spark. But when a tower in the shutter wheel blocks the light beam from the LED; the photosensor senses there is no light, and signals the electronics in the module to turn on the charging current through the coil; so that it will be ready to produce the next spark.
This sequence of charging the coil and then interrupting the charging current to produce sparks is repeated thousands of times a second while the engine is running. And that stream of sparks is passed in the proper sequence to the spark plugs in the engine's cylinders, through the distributor cap, rotor, and plug wires.
What the instructions are saying is for you to measure the voltage at the coil negative terminal with a digital multimeter. They want to know what the voltage is at that point when the charging current through the coil is turned on (when the light beam is blocked). If you measure 2 volts or less at the coil's negative terminal when the light beam is blocked; this test verifies that the module is conducting charging current as it is designed to do. If there was a bad ground connection at the distributor or module, or a defective component in the module; the voltage would be higher than 2 volts at the test point when the light beam was blocked.
If one of the towers in the shutter wheel happened to be blocking the light beam when you wanted to make this test; you wouldn't have to do anything else to block the light beam. But if there happened to be an open slot in the shutter between the LED and the photosensor at that time; you would have to block the beam with a piece of plastic, or some other material which light cannot pass through, in order to make the test.