This block computes the output y as a linear combination of the input u and of its past values (= FIR filter):
y(i) = a[1]*u(i) + a[2]*u(i-1) + a[3]*u(i-2) + …
where y(i) and u(i) are the values of y and u at clock tick i and a[:] are the filter coefficients.
At the first clock tick i=1 the past values are filled with u at this clock tick (= steady state initialization).