What is the meaning of "BOM header information output of UTF-8 "?

What is the meaning of "BOM header information output of UTF-8"? what is the meaning of "BOM header information output of UTF-8, UTF-8 encoding header will have two special characters by default, these two special characters is bom, look at this explanation: http://hi.baidu.com/%CE%D2%B5%C4%B5%C6 "BOM header information output" what does this sentence mean?
What does "BOM header information output of UTF-8" mean?

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The UTF-8 header contains two special characters by default. the two special characters are bom.
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Let's look at this explanation:
Http://hi.baidu.com/%CE%D2%B5%C4%B5%C6%C5%DD%C1%C1%C1%CB/blog/item/9c1660ee5db2b03cadafd5e7.html
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I remember if PHP was stored in the UTF-8 BOM format, there would be a blank line at the top of the page (at least I was doing this under IE8)
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So you will like notepad ++
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It is best not to save the utf-8PHP file as a BOM header of the UTF-8.
Otherwise, you will encounter errors when using session, header, and compressed output in the program.
The BOM header is only a File ID, which is added to the beginning of the file. An ordinary editor is invisible, but it does exist and will be output at the beginning. It is because it is in the header and will be output at the beginning, it will cause errors when session, header, and compressed output are used.