The more light you shine on the phototransistor (the tiny clear box on the module), the larger the output. This allows you to use light to control things like LEDs and Buzzers.
The Photo Sensor module contains a phototransistor that acts much like a normal transistor. In this module, current flows to the base when light hits the sensor, turning the transistor on and allowing current to flow from the collector to the emitter (arrow symbol). If there is no load connected at the output (and the transistor is on), the resistance at the output will be infinite and current will flow through the 10k resistor to ground. If there is a load connected at the output, some of the current will flow through the load at the output, while some will flow through the resistor. The more light shined on the phototransistor, the better it works as an amplifier, and the larger the outputted signal at the emitter.