Corner detection using Chris Harris & Mike Stephens
Harris Corner detector tries to quantify the local intensity changes at all the directions for each pixel. The figure below illustrates the basic idea clearly:



So $I(x+u,y+v)$ indicates the pixel intensities of all the neighborhood pixels around $(x,y)$.
The window function is applied for feature localization. For most often used Gaussian function, the choice of sigma value corresponds to different feature width in the image.