Logic Levels
bri_huang 
Active-Low and Active-High
When working with ICs and microcontrollers, you'll likely encounter pins that are active-low and pins that are active-high. Simply put, this just describes how the pin is activated. If it's an active-low pin, you must "pull" that pin LOW by connecting it to ground. For an active high pin, you connect it to your HIGH voltage (usually 3.3V/5V).
For example, let's say you have a shift register that has a chip enable pin, CE. If you see the CE pin anywhere in the datasheet with a line over it like this, CE, then that pin is active-low. The CE pin would need to be pulled to GND in order for the chip to become enabled. If, however, the CE pin doesn't have a line over it, then it is active high, and it needs to be pulled HIGH in order to enable the pin.
Many ICs will have both active-low and active-high pins intermingled. Just be sure to double check for pin names that have a line over them. The line is used to represent NOT (also known as bar). When something is NOTTED, it changes to the opposite state. So if an active-high input is NOTTED, then it is now active-low. Simple as that!