RCLK (Register Clock / Latch) is a very important pin. And bits are transferred in on the rising edge of the clock. This means that in order to shift bits into the shift register, the clock must be HIGH. The 595 is clock-driven on the rising edge.

SRCLK (Shift Register Clock) is the clock for the shift register. SER (Serial Input) pin is used to feed data into the shift register a bit at a time. VCC is the power supply for 74HC595 shift register which we connect the 5V pin on the Arduino. GND should be connected to the ground of Arduino. You can understand this better with the illustration shown below. Each bit of the Storage Register is connected to one of the output pins Q A–Q H of the IC, so in general, when the value in the Storage Register changes, so do the outputs. On enabling the Latch pin, the contents of Shift Register are copied into the second register, called the Storage/Latch Register. At the rising edge of the pulse, if the data pin is high, then a 1 gets pushed into the shift register.

Bit 0 in the Shift Register accepts the current value on DATA pin.For example, Bit 7 accepts the value that was previously in bit 6, bit 6 gets the value of bit 5 etc. The bits in the Shift Register move one step to the left.Whenever we apply a clock pulse to a 595, two things happen:

The Shift Register lies deep within the IC circuits, quietly accepting input. The first one is called the Shift Register. The 595 has two registers (which can be thought of as “memory containers”), each with just 8 bits of data.

Serial shift register