74HC595 is an 8-bit shift register IC. You can use it to control 8 outputs at a time with only a few pins from your microcontroller.
You can chain the IC as many as you want so that you can control more output. For example, if you chain 2 ICs, you will get 8x2=16 output pins that you can work on while using the same number of pins from your microcontroller. This IC is popular to control LEDs where many control pins are required.
