A shift register lets you connect up to eight digital signals to your circuit using only three inputs. You could control, say, 8 LEDs with three pins on an Arduino. But the fun doesn’t end there! These 74hc595n shift registers daisy chain. You can connect Arduino to shift register 1 to shift register 2 to shift register 3 and so on, each one giving you eight more pins to control and still only using three pins on your Arduino. Sparta!
The shift register also works in reverse: with only three pins you could read in 8 digital signals per register. A shift register could even control other shift registers! The register also has a master reset switch to turn off everything at once,
If you need to send or read analog signals, consider using a multiplexer.
Check out these videos for more