Those white lines are called contrails (short for condensation trails).

Contrails are like man-made clouds.

Plane engines release hot, moist air with tiny soot particles from the fuel. When this mixes with the high-altitude cold air, water condenses around the particles and forms tiny droplets and ice crystals.

Contrails only form when the air is cold and humid. Even though planes fly at around 35,000 feet where the air is very cold, contrails only form about 5-10% of the time. Depending on the air, they can last from a few seconds to several hours.

Contrails are not directly harmful, but they do trap heat in the atmosphere and can play a major role in adding to global warming.