For example "
" starts a new line in the HTML code, and its results are only seen if you look at the “source HTML”. It is NOT the same as <br>!

The quote above is from manual-Introduction
What does

is exactly what your quote says it means, it starts a new line in the code so for example if i was to type

echo "<html><head></head></html>";

That would all show on the same line, but if i was to add
to the line like below

echo "<html>\

This will now put <head>, </head> and </html> all on there own lines.

this explanation might help

\ is one of a number of escape sequences, and it means insert a linefeed character at this point. Others examples are \ (horizontal tab character) and \\" (to print " within a double-quoted string).

For more, see the manual page on double-quoted strings.