PIR sensors are used to detect motion from pets/humanoids from about 5 meters away (possibly works on zombies, not guaranteed). This sensor is much smaller than most PIR modules, which makes it great for unobtrusive projects. It's also fully contained and very configurable. After you've provided power and ground, you can use the remaining 3 pins to configure whether the sensor is enabled (OEN), how sensitive the PIR should be to motion (SENS) and how long the output is active for (ON TIME). Even though there are 6 pins, the wires are flexible and long so the sensor is breadboard or perfboard friendly.