Shift registers are designed to pass a bit or bits through the Maximum register size.
the Clock / or pulse instructs the process to move the bit.
EG - 4 bit reg size
0000
turn on first bit
0001
now pulse one bit
0010
and pulse again
0100
add first bit again
0101
and one pulse
1010
add one bit again
1011
and pulse
0110
note the highest bit drops off