You open a CSV and the very first thing in it is , sitting in front of a column name that was supposed to be id. Or you import it, and a script that has worked for a year suddenly cannot find that column at all.
What is  at the start of a CSV file
A UTF-8 byte order mark, read the wrong way
Three bytes — EF BB BF — that mark the file as UTF-8. A reader that understands UTF-8 sees one invisible character and shows nothing. A reader that assumes one byte per character sees three, and draws them as . The file is fine; the table being used to read it is wrong.
U+FEFF through TextEncoder, and the bottom row by reading each one as a single byte — the same two operations your editor and your spreadsheet are performing. Nothing in the file is broken. One of them is using the wrong table.Why anybody writes it on purpose
Excel on Windows opens a double-clicked CSV using the system code page, not UTF-8 — unless it finds the byte order mark at the front, which tells it otherwise. Without the mark, a file full of Arabic or accented names comes up as a screen of Ø and Ã. With it, the same file opens correctly on a double-click.
So every exporter that expects its output to be opened by a human on Windows writes one, and is right to. The mark is not a mistake or a leftover. It is one program leaving a note for another, and the note is only rubbish to whoever cannot read the handwriting.
The failure that costs money
It attaches itself to your first column name. A parser that does not strip the mark hands back a header of <invisible>id rather than id. On screen the column reads id. In a lookup, a join, or a config that names the column, it matches nothing.
Nothing errors. The import succeeds, the row count is right, and one field comes back empty forever. This is worth naming because the visible version of the problem —  in a cell — gets noticed and fixed in a minute, and the invisible version gets blamed on the data for a week.
Keep it or strip it
There is no universally correct answer, only a question about what reads the file next.
Keep it when a person is going to double-click the file on Windows, and especially when it contains anything outside plain English. That is the one case the mark exists for.
Strip it when the file is going into a script, a database import, an API, or any pipeline that told you what encoding it expects. Those readers know the encoding already and the mark is only a stray character on the front of your first field.
Removing it changes nothing else. It is three bytes at the very start; every other byte in the file is untouched. The encoding fixer reads the file in your browser, and the CSV cleaner will show you the parsed header so you can see whether the mark is still riding on it.
Why UTF-8 has one at all
It is inherited. In UTF-16 a byte order mark does real work: it tells you whether each pair of bytes arrives big end or little end first, which is genuinely ambiguous. UTF-8 has no such ambiguity — the byte order is defined by the encoding — so the mark carries no information about order.
What survived is the signature. Three bytes at the front that say this is UTF-8, which the Unicode standard recommends against and which too much software still needs. For more on what the encoding itself is doing, UTF-8 explained covers the part underneath this one.
The short version
 is a UTF-8 signature seen through the wrong reader. Keep it if a human on Windows will open the file; strip it if a machine will. And if a column lookup has started failing for no reason, check the first header for a character you cannot see.