Stellarwind was the name given to a program by the United States National Security Agency (NSA) to collect information and conduct surveillance on electronic communications of people outside the United States.
Imagine there is a spy agency that wants to know what people are talking about. These people are not from their own country, so they cannot just spy on them as easily. So, they make something called Stellarwind.
Stellarwind is like a big net that the spy agency puts over the internet to find and collect information. They use software and computers to look at emails, and listen to phone calls to see if people are talking about something they want to know.
The spy agency uses Stellarwind to collect a lot of data about the conversations people are having. They then look at this data to see if it contains any useful information they need.
However, some people think Stellarwind was not a good thing because it could collect data on people who were not doing anything wrong. It could also collect data on people who were inside the United States, which is against the rules. So, some people were concerned that Stellarwind might invade people's privacy.