The concept of spatial frequency has come up several times here on the Studio Artist user forum. And i know that many of you have difficulty conceptualizing spatial frequency. So i wanted to post this picture which i think gets across the concept in an easy to visualize way.
The bottom right corner image shows a series of vertical sine waves that increase in spatial frequency. So as you move from left to right in the image you are sweeping through low to high spatial frequencies. The intensity of the luminance difference in the sine waves decreases as you move from the bottom to the top of the image. What's cool about this is that you can see how your visual system perceives different spatial frequencies.
Note that you are more sensitive to viewing spatial frequencies in the middle of the sweep from left to right, which means that you have more less sensitivity to low and high spatial frequencies. Compression codecs used to compress photos or video take advantage of this fact by using less information to store high and low spatial frequencies in an image they are compressing.
This simple example only shows spatial frequency at a single angular orientation. Real world images and textures will typically have energy at a variety of different spatial frequencies and angular orientations. Low spatial frequency in a real world image corresponds to the mass of the image, while high spatial frequencies correspond to the edges of the image.