Parasitic Drain in car batteries refers to the consumption of power even when the ignition key is turned off. A small quantity of power is consumed even when the car engine is switched off.
This is because power is consumed by the computer in the engine, the car alarm and device like the clock. This may not be much. But if the car is kept unused for long periods, the battery will drain.
The car may then have to be jumpstarted with another battery.
While this is normal parasitic drain, abnormal parasitic drain can cause the battery to lose power overnight. This may be due to a light that you forgot to switch off. Parasitic drain can also be due to leakage due to a ground even though a fuse is more like to blow in such a scenario.
If you suspect that there is a parasitic drain, you can measure the current of the battery and then remove the fuses one by one. This will help identify the circuit which is causing the drain.