It's a character in unicode: U+FEFF, the Byte Order Mark.
It's special and invisible, but it can be useful
But it does at least confirm "this is a UTF-8 text file", since UTF-8 can look like ASCII.
Most text editors know that this character is invisible and shouldn't be rendered, which means you can have BOMs in your files and not know it.
"Actually, I think that the UTF-8 BOM is a deeply stupid idea that serves no useful purpose in any imaginable universe. We wouldn't be thinking about were it not for the fact that MS Notepad happens to write one for UTF-8 documents."
"The Unicode Byte Order Mark: It's The BOM, Yo!"