Parallel charging lets you charge multiple LiPo batteries at the same time, on the same charger. Parallel charging is hugely useful if you have a high-wattage charger and lots of small batteries, such as the 1300 mAh packs that are commonly used for drone racing or fpv acro flight. Parallel charging lets the charger "see" the many small packs as if they were one big pack, and charge them at a high amp rate without damaging them.
But parallel charging has its risks. FIRE! EXPLOSIONS! DEAD KITTENS! Okay, not the last one.
This video will show you how to safely parallel charge LiPo batteries. If you follow the tips in this video, you can parallel charge without damaging your batteries, and without burning down your house.