By default our minds blur and average the information that our eyes receive. When you look at a photograph, your mind is really only processing core elements, shapes, patterns. All of the other core design principles deal with the physics of design, but contrast deals almost entirely with optical recognition. Beauty is in the eye of the beholder, and contrast tends to behave the same way. As such, the potential and successes or failures of this principle tend to be more extreme.
Take for example contrast in color. Color alone can be incredibly powerful to separate and highlight elements of a design, but also brings in a variety of physical and psychological challenges, especially when trying to make a universally admirable design. The Ishihara color-blindness tests are a great example of the complexities faced with contrast. Where a majority of people would recognize the differences between colors, we all perceive color slightly differently. Thus, with this example of how colors alone can be a point of failure in contrast, you should probably learn some basics subcategories that help with contrast in design:
Hue: is the measure and use of color, which can be used to create subtle or very harsh contrast between elements. As typography was mentioned as a large topic of discussion by itself, so too is color theory. As it relates to contrast, it is worth noting that there are many different types of color wheels that map both the optical and psychological effects of using various colors together. By itself, color should be seen as a less desirable attribute to create contrast.
Value: is the measure of light and dark in a design. Obviously pure black and white are the most extreme in value contrast. Before attempting to use color, it is often valuable to design in grayscale first in all mediums. This allows you to evaluate and use values, which are more consistently perceived by the human eye and mind than colors alone.
Texture: in both the digital and physical world, texture gives a sense of depth. Depth can be a great way to create contrast and make an element stand out. A great example of this is actually braille! Design matters not just for the seeing, but is extremely relevant in so many contexts. When you consider that texture alone is powerful enough to represent entire alphabets and languages, you start to get a sense for how meaningful texture can be in design. However, as a word of caution on the digital side, texture can add complexity very quickly and should be used only when actually necessary.
Shape: just like the physics in other design principles, shape relates to our understanding of the physical world, and as such is a great tool for contrast. Intentionally using an organic shape against a rigid shape, for example, can convey a sense of tension or by directly impacting our perception of balance.