Need help to understand the code

Here is the code by P. Valko and J. Abate for finding numerical inverse Laplace transform by trapezoidal’s rule. Can someone willing to explain this code? I don’t understand this part: Plus @@ Append

talbot[F_, t_, M_: 32] :=
     Module[{np, r, S, theta, sigma},
      np = Max[M, $MachinePrecision];
      r = SetPrecision[2 M/(5 t), np];
      S = r theta (Cot[theta] + I);
      sigma = theta + (theta Cot[theta] - 1) Cot[theta];
      (r/M) Plus @@ Append[Table[Re[Exp[t S] (1 + I sigma) F[S]],
                  {theta, Pi/M, (M - 1) Pi/M, Pi/M}],
                     (1/2) Exp[r t] F[r]]]
fun[s_] = 1/(s - 1); %example use
t0 = 1;
ft = talbot[fun, t0]