Package iter provides a syntactically different way to iterate over integers. That's it.
This package was intended to be an educational joke when it was released in 2014. People didn't get the joke part and started depending on it. That's fine, I guess. (This is the Internet.) But it's kinda weird. It's one line, and not even idiomatic Go style. I encourage you not to depend on this or write code like this, but I do encourage you to read the code and think about the representation of Go slices and why it doesn't allocate.