What is the function of the latch pin on my 74HC595 IC?
A positive edge (that is, an input going from low to high) on the &quot;latch&quot; (RCLK, pin 12*) pin of the 74HC595 transfers the 8 bits of data that have been shifted in via the SER pin (pin 14*), from the shift register to the parallel output storage register that controls the 8 parallel outputs (when /OE is low).
This is done so the output of the 74HC595 remains stable, while shifting in the next 8 bits of data to be output. This also allows multiple 74HC595s to be cascaded, while being able tp simultaneously update the output of every 74HC595 in the chain.
The 74HC595 Data sheet explains how this works.

(From the Texas Instruments 74HC595 Data Sheet)
*DIP package