Fragrance load tells you how much of your candle is fragrance oil versus wax. A 10% fragrance load means 10% of the total candle weight is fragrance oil.
Different waxes can hold different maximum loads. Soy wax typically holds up to 10%. Paraffin can hold up to 12%. Beeswax is lower, around 6%.
Exceeding the maximum load causes problems. The wax can't hold all the oil, so it seeps out (called sweating). The candle may not burn properly. The wick can mushroom or produce soot.
Fragrance load is calculated by weight, not volume. Always use a digital scale when measuring fragrance oil.