Over time, charged ultracapacitors without a load slowly lose their charge. "Self-discharge" is the voltage drop a charged ultracapacitor cell experiences after a set period of time without a load.
Ultracapacitors require a current to keep them charged. The leakage current is the stable parasitic current needed to hold an ultracapacitor on charge at a rated voltage. The value needed for the leakage current is voltage- and temperature-dependent.