pre-processing of an image means "preparation" of the sample/image to introduce it to an algorithm for specified task : tracking targets , recognition, feature extraction, etc .
An example, for pattern recognition : you have set of images for testing/recognition process (like face recognition) : the preprocessing is the size adjusting of the considered image, luminance normalization, statistical normalization ( all the samples having mean 0 and variance 1 ), filtering noise with specified filter ( Gaussian kernel, median, Kalman filter, low-pass filter ....), conversion to certain class, etc
In some cases, some algorithms may be used for " preprocessing" like the PCA ( Principal Component analysis) to reduce the samples' dimensions.
