This document discusses image filtering techniques. It explains that image filters are sets of digital processing functions designed to highlight or suppress specific features in an image based on their spatial frequency, which refers to the frequency of tone variation over a small area. There are three main types of filters: low-pass filters, which smooth images and reduce details; high-pass filters, which emphasize details and are used to sharpen images; and directional filters, which highlight linear features oriented in a specific direction. The document provides examples of each type of filter and their effects on images.