Superfatting is adding in more oil than is required to have a complete 1:1 reaction with the lye in a batch of cold process soap.
The small amount of extra oil left over in the soap adds extra moisturizing qualities to the soap. It also gives you a bit of a "fudge factor". (Having extra lye in your soap is a much larger problem than having extra oil.)
For a more complete explanation visit How and Why to Superfat Your Soap
Also Known As: Lye Discounting--(not to be confused with water discounting)
"Superfatting percentages are usually in the 3-8% range."
"I'm going to superfat this recipe at 5%."
"I did a 5% lye discount in this recipe."