Most graphic designers and web people understand RGB to mean red green blue. When combined these three colors create all the millions of colors you see on your computer display.
On the web you might work on a website and desire the hex value for red or green. You're familiar with the RGB value of 255, 0, 0. But you don't know how to quickly find out the equivalent hex value. There's an easy way to figure this out:
Let us go back to the basics. In RGB a value of 255, 255, 255 equals pure white. 255 means a maximum color while 0 equals no color. So to get a color like red the RGB value would be 255 (maximum red), 0 (no green), 0 (no blue).
To understand hex values think of three pairs of letters/numbers, like RGB, but instead of numbers the values move from 00 to FF. In hex values 00 means no color while FF means maximum color. If you have three pairs like FF FF FF this means red (first FF is maximum), green (second FF is maximum) and blue (third FF is maximum). All three equals pure white. So if you wanted to quickly find out what the hex value for red is think all red, no green, no blue... FF 00 00. That's it!
Web and graphic design share the same basic principle of three values. The difference between RGB and hex is the spacing and values. Also RGB values are generally comma separated by spaces, while hex values are all sandwiched together with a hash symbol in front of the value.
Red in RGB: 255, 0, 0
Red in hex: #FF0000
Hope this helped!