A thread is basically a particular path of execution of code. Thread1 is the “main thread”, the one where your app does its basic operations, all of the user interface code, etc. When you start getting into more sophisticated programming, you might employ concurrency, where you send time consuming operations to a background thread/queue so that it doesn’t adversely affect the user experience which is happening on the main thread.
The threads 2, 3, 4, etc. in your question, though, are system generated threads and not ones you generally need to concern yourself with. Only worry about the main thread and, if and when you get into writing concurrent code, those threads to which you are explicitly adding operations.
https://developer.apple.com/library/archive/documentation/General/Conceptual/ConcurrencyProgrammingGuide/Introduction/Introduction.html
https://stackoverflow.com/questions/15764108/debugging-by-thread-in-xcode
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