A pack of 5 74HC595 Shift registers.
Arduino has a tutorial that can explain it better than we can: Arduino "ShiftOut" tutorial
In short though, it allows you to convert a serial signal (8 bits of data coming in on one wire) to 8 parallel outputs.
The serial signal uses 3 pins - one for the data, one for the latch, and one for the clock.
So you'd send it something like 10001010 on the data line, and then on the clock signal, it will output HIGH to pins 1, 5 and 7.
You can see why 1, 5 and 7, right? Because those are 1s in the signal received on the input.
Link to Datasheet