Superfat is the percentage of oils in your soap recipe that don't get converted to soap during saponification. They remain as free oils in the finished bar.
A 5% superfat means 5% of the oils are left over after the lye has reacted with the rest. This makes the soap milder, more moisturizing, and gentler on skin.
Most cold process soap recipes use 3 to 8% superfat. Going higher makes the soap softer and can reduce lather. Going lower risks a lye-heavy soap, which is harsh and irritating.
Superfat also acts as a safety margin. If your measurements are slightly off, the extra oil ensures there's no unreacted lye in the finished soap.