A LiPo is a lithium-polymer battery pack, the standard power source in electric RC racing. Cells run about 3.7–3.8V nominal and charge to 4.20V each — 4.35V for high-voltage packs — and must be balance charged.
Per Reedy’s LiPo safety sheet: always use the charger’s balance mode, never charge above 2C, always charge in a fireproof container, and never leave a pack charging unattended. Charge on concrete with nothing flammable within 10 feet (3m). Stop using or charging a pack immediately if it balloons, swells, deforms, leaks or gives off an odor, and don’t exceed 160°F (71°C) in discharge or handling — a puffed pack does not get “one more run.” Store packs around 3.8V per cell rather than full. Reedy’s chart puts the lowest discharge voltage at 3.2V per cell (6.4V on a 2S), and Reedy says any cell under 3.0V should be removed from service. At tech, ROAR checks pack voltage: a maximum of 8.40V for a 2S pack.
A LiPo that's overcharged, over-discharged, or charged while damaged is a fire hazard rather than just a dead pack, which makes this the one piece of RC gear where cutting a corner can burn down the trailer.