We don’t see many planes overhead these days, but the last couple of afternoons we have seen a condensation trail as a plane has flown over.

Condensation because water vapour in the atmosphere condenses out on soot from the plane engines as the plane flies through the cold moist air. It makes a cloud that can hang in the sky for ages or dissipate quickly. In this case it was almost gone by the time the plane had passed over.

There are conspiracy theories that the trails are releasing chemicals into the atmosphere. This isn’t true but stems from some experiments years ago by scientists seeding clouds with silver iodide to create rain by giving water droplets a nucleus for the water droplet to form on.

So there is a grain of truth, but just that. The reality is far simpler. See https://en.m.wikipedia.org/wiki/Contrail. For more information.