BOM is a Byte Order Mark, used by unicode encodings such as UTF-8. It's some bytes added at the beginning of the file which text editors use to distinguish the encoding and to know the byte order. UTF-8 does not require it because it uses multibyte characters. When PHP includes the language file which has a BOM (added from your text editor) it thinks the bytes at the beginning of the file are characters, and as they are outside of the <?php block, it outputs them. That results in the HTTP headers being sent to the browser, which means phpBB is unable to send it's own. Which results in the error you are getting.
Short version: disable the "Unicode BOM" setting in your text editor to fix the error