Hash mark prefix to a string

I asked this question on the Pike channel on libera.chat but nobody
is lurking at the moment. :)

what does the # mean herein the following?

[code]
str =3D
#"This is a multiline string
terminated by a double-quote like any other string";
[/code]

I've searched all the available docs that I can find, but have yet
to stumbled on an explanation of this syntax. TIA ..
=2D-
Duke
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