Spatial Averaging results in smoothing(blurring) of an image.
Each pixel is replaced by the weighted average of its neighborhood pixels in a 3x3 or 5x5 window.
An example of a 3x3 window for averaging is :
[ 0 1/8 0 ]
[ 1/8 1/2 1/8 ]
[ 0 1/8 0 ]
If the centre pixel is reduced to 1/4, the brightness of the image will be reduced as well.
Spatial High Pass Filtering (Sharpening)
High Pass filtering will sharpen an image by enhancing the edges and high frequency details.
An example of a 3x3 window for sharpening is :
[ 0 -1 0 ]
[ -1 5 -1 ]
[ 0 -1 0 ]