Quoting
Shell Syntax
In a non-interactive shell, or an interactive shell in which the
interactive_comments option to the shopt
builtin is enabled (see Bash Builtins),
a word beginning with `#'
causes that word and all remaining characters on that line to
be ignored. An interactive shell without the interactive_comments
option enabled does not allow comments. The interactive_comments
option is on by default in interactive shells.
See Is This Shell Interactive?, for a description of what makes
a shell interactive.
Quoting
Shell Syntax