According to dictionary.com, the word contrast means "to compare in order to show unlikeliness or differences." Now, although this is a broad and general definition, it essentially captures the meaning of what contrast is in a visual context. In a visual context, contrast is the relative differences between light and dark areas or filled and un-filled space. Contrast, used correctly, captures a person's attention quickly and draws them into an image. Designers and photographers can use contrast to their advantage, however, too much or too little contrast can quickly become overpowering or underwhelming respectively.
In my neighborhood, I found countless signs in which contrast plays a valuable part. Below I will show an example of one that I believed used contrast well and one that did not:
This sign, I thought, could have used the concept of contrast more effectively. Despite being black and white, the highest color contrast, the sign does not quite capture one's attention immediately. Unlike other signs of this nature, the lettering is not very bold and much of the sign is un-filled space. This abundance of space cause the words to have less of a powerful effect.
To me, this sign was a prime example of how to communicate a message effectively using the idea of contrast. First of all the sign simply uses three colors: red, black, and white. The black and white create solid contrast while the red does an excellent job of drawing in one's attention. Additionally, the print is in bold, making it easy to read. There is also little unused space, especially with the simple but communicative image of the surveillance camera.