You can either use imfilter() in conjunction with fspecial() to generate the filter kernel, or in the specific case that you want a gaussian kernel, you can use imgaussfilt().
As to what it does, there should be examples at the given links, otherwise:
inpict = imread('cameraman.tif');
% use imfilter
fk = fspecial('disk',5); % a flat circular filter
A = imfilter(inpict,fk,'replicate');
imshow(A)
% use imgaussfilt()
B = imgaussfilt(inpict,3);
imshow(B)
