The vapour trails (or "contrails") expelled behind aircraft - a result of the heat from the engines condensing water droplets suspended in the atmosphere and leaving a trail behind them as they pass - are claimed by some as a deliberate spraying of undetermined 'chemicals' by 'the government'. For what nefarious reason this is being done is confusing. Some say it is to cause/mitigate specific weather, some say it is to sterilise men and some say it is turning frogs gay.
The whole chemtrail conspiracy is a modern internet spread meme that seems to ignore the fact that planes have been leaving vapour trails since flying at speed was realised. The whole thing seems to have started in the 1990's following reports of military experiments attempting to control local weather patterns.
Photos on the internet showing water tanks inside otherwise empty aeroplanes are correct, however these are usually water tanks used to simulate passenger/cargo weight in testing new and modified aircraft.