doubt in specific line

This is the input

mask_h=2;
mask_w=4;
mask = zeros(mask_h,mask_w);
mask(1:mask_h/2,:) = -1;
mask(mask_h/2 + 1:end,:) = 1;

kindly explain the following line. What replicate means and how it will work?

img_filt = imfilter(img, mask,'replicate');

Answer 1

[Walter Roberson]

'replicate'

Input array values outside the bounds of the array are assumed to equal the nearest array border value.

(from doc imfilter )

[Rd]

I can't understand exactly...could you please briefly explain?