A sloper is a basic pattern for a garment based on body measurements--as Wendy said, without seam allowances. So if you want to make your own clothing patterns without purchasing a pattern, you make the sloper, a basic cardboard pattern that reflects your measurements. Then, for the different garments, you make a pattern from the sloper with tissue paper, by adding features, such as necklines, darts, etcetera and it will be designed specifically for your body, whereas with a store-bought pattern, you are most likely to have to make modifications to the pattern.
I wish I had done it when I was younger because then I would have been making all sorts of clothing--I did go back to my mom last year and got her to teach me how to modify a pattern and in the process she taught me how a sloper is developed. It was easier for me to understand how to make a pattern from sloper than it was to modify a store-bought pattern. I wish I had learned when I was younger, now I have a half-baked idea of what it entails but still can't do it.
Wendy, a tailor's dummy would be fantastic--I can never imagine getting a garment done because I could never do a proper fitting by myself. That would solve the problem, wouldn't it?