circshift(s,d) for a vector s and positive integer d shifts the elements of s to the right by d amount, wrapping back to the beginning when they go off the end (thats why it's called circular). If d is negative, elements are shifted to the left by -d amount.
It's easier to see what it does with a few examples in MATLAB rather than a few sentences in English.
In your code, the first 4 elements of s5 are shifted left by one place and stored back into s5 as the first 4 elements (and the remaining elements are untouched):
s5(1:4) = circshift(s5(1:4),-1)