Means padding so that output size is the same as the input size. $(n + 2p) \times (n + 2p) * f \times f = (n + 2p - f + 1) \times (n + 2p - f + 1)$ In order to make the output size the same as the input size: $n + 2p - f + 1 = n \Rightarrow p = \frac{f - 1}{2}$