Shift registers store a sequence of binary digits ('bits'). The length of the sequence is a fixed property of the shift register. When the clock pin ('>') sees a pulse the bits are shifted along (the bit at stage n is moved to stage n+1). In this process, the bit at the last stage is lost, and the bit in the first stage is read from the serial input pin (usually labelled 'S').
Normally not all bits in the sequence are displayed on outputs. In the minimum case there is a serial output pin, which displays the last bit in the sequence.