What does the noeol indicator at the bottom of a vim edit session mean?

Unix editors like vi and vim will always put newlines (\n) at the end of every line - especially including the last line. If there is no end-of-line (eol) on the last line, then it is an unusual situation and the file most certainly was not created by a standard UNIX editor.

This unusual situation is brought to your notice by the [noeol] flag in the vim editor; other editors probably have similar flags and notifications.

That the last line in the file doesn't have a newline (\n)