Imagine you are in a room with a lamp. If you turn on the lamp and walk around it, the light can follow you everywhere. But what if you add some smoke to the room? The light will still shine, but it will be harder to see and it might look fuzzy or hazy. That's kind of what happened in Southeast Asia in 2015, but with air instead of light.
In Southeast Asia, there are a lot of forests and farms where people grow things like palm oil, rubber, or paper. Sometimes, people burn parts of the forest or farms to clear the land for new crops. It's a cheap and easy way to get rid of old trees or plants, but when many people do it at the same time, the smoke can travel a long way.
That's what happened in 2015. Lots of people in Indonesia started burning things and the smoke spread to other countries like Malaysia and Singapore. The smoke was so thick that people had trouble breathing or seeing well. Schools had to close and people had to wear masks to protect themselves from the smoke.
The smoke also had an impact on animals and plants. Some animals couldn't find food or water because the haze made it harder to see or smell. Plants couldn't get enough sunlight and nutrients because the smoke blocked them out. Even airplanes had to be careful when flying because the smoke made it harder to see or navigate.
In summary, the Southeast Asian haze in 2015 was caused by many people burning things, which produced smoke that spread and made it hard for people, animals, and plants to breathe, see, or grow.