74HC595 is a displacement register with 8-bit serial input and parallel output: the parallel output is a three-state output.
At the rising edge of SCK, the serial data is input by SDL to the internal 8-bit shift register and is transferred by Q7' Output, and parallel output is the LCK rising edge to the 8-bit shift register data into the 8-bit parallel output register. When the control signal of the serial data input OE is low enabled, the output value of the parallel output is equal to the value stored in the parallel output register.