Max has more important things to do!
A little more searching teaches me that it is a marker placed at the beginning of a text stream (which is what a web page really is, despite what it looks like when rendered on your browser), that tells how the bytes are arranged. Hence "Byte Order Marker."
If you're still reading, byte order has to do with how computers represent numbers greater than 255 (one byte worth), by putting the high-order (place value if you will) byte first or last. If the high order byte comes first, they call it Big Endian, otherwise Little Endian. I am not making this terminology up.
Rather than make the receiving program figure out from the data what the Endianness of the stream is, they put a Byte Order Marker at the beginning of the file.
If and when your browser figures out that the first few bytes are in fact this marker, it will commence to display the page as you expect to see it. Until then, it interprets it stupidly as "ï»¿".
I do understand the explanation, actually, now that I've seen it.
I shall now go sing a song of celebration to myself to justify this thread being above the line.