Stearic acid is also known as
stearine. This is a
long chain fatty acid that is an
animal fat byproduct.
With candle making, it is used to modify the melting point of wax. Unless the wax is very 'high grade' (higher melting point) wax, adding steric acid will raise the melting point. By itself Steric acid has a melting point of 160 degrees Fahrenheit (paraffin wax has a melting point between 120 and 140 degrees Fahrenheit, beeswax has a melting point of 145 degrees Fahrenheit). When combined with wax it forms a strong crystalline structure that makes the candle harder and stiffer. Typically it is used in about 2-9 tablespoons per pound of wax.
Steric acid also makes the wax more opaque and less likely to drip. Candles can be made completely out of steric acid. These candles are often used in churches where long burning candles that do not drip or smoke excessively are required.