If you have a sufficient amount of something, it's enough — not too much, not too little, just right. Goldilocks would be pleased.
Sufficient comes from a Latin verb meaning "to meet the need." If something is sufficient it has met, or satisfied, a need. Enough is often used as a synonym for sufficient, and when something is not sufficient, it is too little to take care of what's needed. Sufficient can, however, also suggest just enough and not an abundance, as in "the money was sufficient for groceries, but we needed more to fill the gas tank."